feat: Don't affect MimeMessage with "From" and secured headers from encrypted unsigned messages

If a message is encrypted, but unsigned:
- Don't set `MimeMessage::from_is_signed`.
- Remove "secure-join-fingerprint" and "chat-verified" headers from `MimeMessage`.
- Minor: Preserve "Subject" from the unencrypted top level if there's no "Subject" in the encrypted
  part, this message is displayed w/o a padlock anyway.

Apparently it didn't lead to any vulnerabilities because there are checks for
`MimeMessage::signatures.is_empty()` in all necessary places, but still the code looked dangerous,
especially because `from_is_singed` var name didn't correspond to its actual value (it was rather
`from_is_encrypted_maybe_signed`).

test-data/message/thunderbird_encrypted_unsigned_with_unencrypted_subject.eml

@@ -0,0 +1,91 @@
+From - Sun, 19 Nov 2023 01:08:24 GMT
+X-Mozilla-Status: 0800
+X-Mozilla-Status2: 00000000
+Message-ID: <38a2a29b-8261-403b-abb5-56b0a87d2ff4@example.org>
+Date: Sat, 18 Nov 2023 22:08:23 -0300
+MIME-Version: 1.0
+User-Agent: Mozilla Thunderbird
+Content-Language: en-US
+To: bob@example.net
+From: Alice <alice@example.org>
+Autocrypt: addr=alice@example.org; keydata=
+ xsDNBGOaGzQBDADCFtBNMHRDJQRkd2tNm7CJm1Yo3Y5r3qP6v0FSwP1BIHbiIf0E/jFiKZWj
+ 1uL68J2mGUuUu+Qi4ovf1l9/QQYzg/DCaLZxlbc0LKu2LXcpUL5DPu37mdw+DKs0YvNIlc+A
+ RjyFUwd3rsZN3k58inf1mYzKuKU6NpbdXULbOEYwnVEwzQsrtS2JgJ+tLSYUvNJeMJXm/cDL
+ XKJSApAyvVVdxxteG8uWcDqWV/HcXuopXLILf3yJF0De11/7G62dHNHuhmtgRLsTN4Q372Q9
+ KNdYEFLHaN91jEzyD/+aHNskATxtcGhppI8OQsU3NzNgHyd8Smzx5oTyZ/6NdhYoh0pKB8yf
+ VAyA69t5fctQRb4+bTwL+sS9KDobQOvcyOMUSccDfUhsWMghwsMCwU4Sz9hIY6dCAfroDAiL
+ vYUfdNJstAqvLf04mZtMmkI7Q2BYLETEgu4KQzQHRQekmOE/3EaSiojNa4ZTVURMdJ9U+I3E
+ q8e6TbOY7Xa4V8krAt/F2wMAEQEAAc0ZQWxpY2UgPGFsaWNlQGV4YW1wbGUub3JnPsLBDQQT
+ AQgANxYhBBSrP2X8J0u9tfp2jCXwByRZ5HriBQJjmhs1BQm7+B4AAhsDBAsJCAcFFQgJCgsF
+ FgIDAQAACgkQJfAHJFnkeuKQ2wwAgDgiCI6bz9PjqE1GoDcy/xQdy+nnYq5pOuHGUndZ7jYK
+ cOqM8LDEaG7GgrFsbs9vGhTA1fyqncM41pB7SmwQ7zBVaMdtHoulEG4RPGVboDaY9tuMOL3/
+ GVxFbovVHyU5Lr1euryNh/0JvMITY0UHaEY1k1M7izYUMyFu8I1ODZ9Iws2trUyU3Omw/sTJ
+ x15zzCsK8Aq+r3JmB+Q33SFSgWr/YWH0dQVIQ0I5iLN2q14oucmLBaKc9EXdRLiu8S8lLSQl
+ nfISJ17GBLmH1YxmPPZ3CRHC6iEKCLR6G9wzhsTPNdK7dRCYR5wTI27RVPLBcSnCKAeTopAJ
+ YskyNndtv0iaNRT7YLOfhrsBAofSjuLegR04CNiqBHtYQ3LO3WKhJ7riRcQ/Ksv0wYkmj1gJ
+ 8myMwA+ybfYrpNqO4devnCvE3Eo5gzeYbvYU2Z17n9y6HAOG9/Tm/daiGEP2ni6iwV0kqLzw
+ eC48R1D75T66PxX/jQooujrTph8+K3ckV/q+zsDNBGOaGzUBDADV+DGgKxvCpfVFuPGrSdRU
+ 06dxowdKOKavO6WGMvN3g/+CFrIsjUFy4S0Soo5ARnLh23i49ZSjacXFpgtZUNV3iGOSOcSE
+ LldLtZk5BV9w/ATqqgu4/LVdNA9rm+o197bIeSQCRTnY/QV6FdKYxVd4NBVH9abZ7t8Tm4qC
+ urZj56MjPCg3fqT+Q6sjxH+nKBrs8s8iCJkYhGBgU3q5W+wrtZ56kI9mxJec62KHpyLZ0rTE
+ xEAeVbChUJOo11vUtJfTrDhI6lhqyr72o/A6bY1OV7WzkxtiBRl35eewQ+RDLJ4yxaNj/XTS
+ UxOz60xNggEfDVtfgfjBZrBbiHXqf8iKVV1ZPGm0ycvXZGYFw2zXLI2PwevhQCm+t4Ywty1h
+ 8l019MYmGadpQgbuA4ZippuzOSzSGMQ+S4uYEzeeymR9ksxVSXn90HEzqC7LdHCcd2IO6rfu
+ g2fuRf258Adfuoh3s8YUlWyXjEHLXKo9SRgGMfGs7qgCOL/ReAwFPtKACvEAEQEAAcLA/AQY
+ AQgAJhYhBBSrP2X8J0u9tfp2jCXwByRZ5HriBQJjmhs2BQm7+B4AAhsMAAoJECXwByRZ5Hri
+ EOkMALtq4DVYX8RfoPdU0Dt6y+yDj1NALv5GefvHbgfuaVT8PaOP0gxCjWrnUDvvJEwP1W3j
+ UXYqDwKP42hiGWsnXk2hbgXbplArgP3H987x7c8bu1wIAmkJ9eLjEM++rbOD4vWbYXRwaDiH
+ LetFJ5tGHDAIfL48NYpz2o3XZ3/O7WdTZphsAcvgPxTC+zU7WkbUl2SQlj0/qwsoD+qe9RYT
+ XhVXR7q7sjcGB4TpeqzRT7YKVLoVNq+bQw2lUX4W561gAYbZvVo/XByfDCoxmkxwuMlSmajj
+ Wy7b9TuT38t1HArv4m/LyVuBHiikX0/MUNBeSSIiKDvTL6NdHTjnZM6ptZvdvW3+ou6ET0pK
+ MGDpk/1NVuMnIHJESRg/SSFV6sElgq38k9wAT2oUqLcYvYI07nHmnuciaGygkCcGt+l2PvAa
+ j4mkQQvMU0cNRDBybk5aKi820oGIJjT7e+5RnD2mYZQdOAbQhDVCHvrfS1I60bsHT1MHqyAa
+ /qMLjKwBpKEd/w==
+X-Mozilla-Draft-Info: internal/draft; vcard=0; receipt=0; DSN=0; uuencode=0;
+ attachmentreminder=0; deliveryformat=0
+X-Identity-Key: id3
+Fcc: imap://alice%40example.org@in.example.org/Sent
+Subject: Hello!
+Content-Type: multipart/encrypted;
+ protocol="application/pgp-encrypted";
+ boundary="------------HVIlnYsnz3YPWPVlor40isrE"
+
+This is an OpenPGP/MIME encrypted message (RFC 4880 and 3156)
+--------------HVIlnYsnz3YPWPVlor40isrE
+Content-Type: application/pgp-encrypted
+Content-Description: PGP/MIME version identification
+
+Version: 1
+
+--------------HVIlnYsnz3YPWPVlor40isrE
+Content-Type: application/octet-stream; name="encrypted.asc"
+Content-Description: OpenPGP encrypted message
+Content-Disposition: inline; filename="encrypted.asc"
+
+-----BEGIN PGP MESSAGE-----
+
+wcDMA/K57StIWPW6AQv/cTRuyZRuX+Qkmxcbo34Df32PL2O3LSuwChdd4pd+WRNU5r08E0eLeFGv
+k/C/2iFrNhgmvsJEnDtTZVc7+qykMeXwwC1f2qT7OAvTbAF9Me1dBCQy8QAPSzOowNu8434qOPwz
+fZkoB4pdkKlAhGMUBdtbWYg3EDdpBUCpAuT8l3qhlfdSTRh+da2UVh9kix3SXSmLarMZGlJikh3T
+pRLAlPOtRa7k5R4rc/OCD9HUL9EHGUk4rV86+mr4Wpp1aIhdTNksfrFXvgRFx0dcU33uS74X/zme
+c1AOuJK8ed3F2Zc5IYTyE+ps1jBqVH1+CM697d5FnIMx96VyB4txDBBBjyq+oh+SY+xUxvtR/EzN
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+NO4DJ6/x2UCcUiUokAwvKTSHflAHocZJ2ICfzhlJjJSiqEttwyG0xU0ZhTdZIbwzRWzYzDgMakFO
+wcBMA+PY3JvEjuMiAQf+MXJB44iQ/Ti9WDH3MOX0N+X0APf5mzeqU9MpSZ+7mAnTRVmz/iMD989J
+ngxu1mv3vQBjdNokIAfBYk29qyRBULXaof+6x5VJSWoopV6t1vNd+DB2HgLkbdCJuzikapCE+QAm
+gmgoknQap+3l4D1RkMys7w1awsMYK0wR1Iucjb8M2I4f5ObPSMS5211ZBJuYOf1OQt0jX1jCNTOi
+Q5tbufJ6EjAvP6XOYTY3in8+p7yocBgQhXaK8NB8jdg+h6IE/NNX1W5v1tMx17WRtIQGwh4cOlB6
+Fsu87eMx8x9Ew2YdpN6yvIHddy9M9k3NCROT+5rIq7+1GTy0WoI/KxNwtdLBEwE2h7VFmJ5qEWhi
+lmunWBIlA71IdNqpi+9lbg/QPwCgvRow05Gv0FyKbSvDA/fN18+CLuz8RicNFRbiPgwxMLRE3lZB
+jj7BEDa53fjnjBJG4lA9mUaB/ALScQwkGXqKPpeDN4Iexy8eBsZoBczJcPna7GNgSYiXxbWo2P8+
+T71oSht+igsyi8gYDNwmhcsQxSWtF7f53irOKWgEpgz2hMjWi764GGBzAJ231suyV2eZXVJm2Im7
+RUDu3bJNu/Q/CvxQQdcUXHEoHrTzDZ3KQPW0/oYln2WZwkRQcVVGSLVINgVTdTkI8GEuXH/4yjYF
+tUq68esMQ2WOWZ7IiTNPU9T2F5kEwR2sf55XgLckj0OLJn5CkSAWBJhHdL/9er06u8ksTs6V9UEo
+i+15XUAiNzsuIcydoGF9sSuO9nm62xYO8uY8yQl/z2Q7CXKefkXBSeaNcYyQKrCnLfwnuvR2WFE5
+OJ0HM5i/qcWmqWSvSYzdDqUbI95q9Fgn78F/bcJ9ZTdPDkWLi//SUzcWeUWgRC57U4CFTApj74EJ
+lT/gVlH/y88RNcvGGAPHHS11adLmln7CbLEF9tmRM3ou78nMm7VhXT8C/QZiTvJgG3yZbeRFlC39
+=hdiO
+-----END PGP MESSAGE-----
+
+--------------HVIlnYsnz3YPWPVlor40isrE--