feat: key-contacts

This change introduces a new type of contacts
identified by their public key fingerprint
rather than an e-mail address.

Encrypted chats now stay encrypted
and unencrypted chats stay unencrypted.
For example, 1:1 chats with key-contacts
are encrypted and 1:1 chats with address-contacts
are unencrypted.
Groups that have a group ID are encrypted
and can only contain key-contacts
while groups that don't have a group ID ("adhoc groups")
are unencrypted and can only contain address-contacts.

JSON-RPC API `reset_contact_encryption` is removed.
Python API `Contact.reset_encryption` is removed.
"Group tracking plugin" in legacy Python API was removed because it
relied on parsing email addresses from system messages with regexps.

Co-authored-by: Hocuri <hocuri@gmx.de>
Co-authored-by: iequidoo <dgreshilov@gmail.com>
Co-authored-by: B. Petersen <r10s@b44t.com>

test-data/message/mixed-up-long.eml

@@ -1,6 +1,3 @@
-From - Tue, 29 Aug 2023 20:24:31 GMT
-X-Mozilla-Status: 0801
-X-Mozilla-Status2: 00000000
 Message-ID: <05eae88e-35c9-5e5e-405f-11e8a3b44513@example.org>
 Date: Tue, 29 Aug 2023 17:24:31 -0300
 MIME-Version: 1.0
@@ -9,10 +6,6 @@ User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Content-Language: en-US
 To: bob@example.net
 From: Alice <alice@example.org>
-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: ...
 Content-Type: multipart/mixed;
  boundary="------------2IZJ0SaOTFMF25fU1nsH7bxg"