Repair encrypted mails turned into attachments

Google Workspace has an option "Append footer" which appends standard
footer defined by administrator to all outgoing messages. However,
there is no plain text part in encrypted messages sent by Delta Chat,
so Google Workspace turn the message into multipart/mixed MIME, where
the first part is an empty plaintext part with a footer and the second
part is the original encrypted message.

This commit makes Delta Chat attempt to repair such messages,
similarly to how it already repairs "Mixed Up" MIME structure in
`get_mixed_up_mime`.

test-data/message/google-workspace-mixed-up.eml

@@ -0,0 +1,78 @@
+Subject: ...
+MIME-Version: 1.0
+Date: Tue, 10 May 2022 08:24:11 +0000
+Chat-Version: 1.0
+Message-ID: <foobar@example.net>
+To: Bob <bob@example.net>
+From: <alice@example.org>
+Content-Type: multipart/mixed; boundary="000000000000b4907a05dea40c88"
+
+--000000000000b4907a05dea40c88
+Content-Type: text/plain; charset="UTF-8"
+Content-Disposition: inline
+
+
+-- 
+
+
+
+
+
+
+
+--000000000000b4907a05dea40c88
+Content-Type: multipart/encrypted; protocol="application/pgp-encrypted";
+	boundary="bi1vjiRd9u4EaEuno1ejliJactdnFl"
+
+
+--bi1vjiRd9u4EaEuno1ejliJactdnFl
+Content-Type: application/pgp-encrypted
+Content-Description: PGP/MIME version identification
+
+Version: 1
+
+
+--bi1vjiRd9u4EaEuno1ejliJactdnFl
+Content-Type: application/octet-stream; name="encrypted.asc"
+Content-Description: OpenPGP encrypted message
+Content-Disposition: inline; filename="encrypted.asc";
+
+-----BEGIN PGP MESSAGE-----
+
+wV4D5tq63hTeebASAQdAt2c3rVUh+l0Ps7/Je83NaA7M6HsobtfMueqLUBaeancw0rRAo7PbLDLL
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+=dcQr
+-----END PGP MESSAGE-----
+
+
+--bi1vjiRd9u4EaEuno1ejliJactdnFl--
+
+
+--000000000000b4907a05dea40c88--