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/thunderbird_signed_unencrypted.eml

@@ -1,6 +1,3 @@
-From - Thu, 15 Dec 2022 14:45:17 GMT
-X-Mozilla-Status: 0801
-X-Mozilla-Status2: 00000000
 Message-ID: <db254c95-c23e-2ad5-14db-11ad1f374dbf@example.org>
 Date: Thu, 15 Dec 2022 11:45:16 -0300
 MIME-Version: 1.0
@@ -10,10 +7,6 @@ Content-Language: en-US
 To: bob@example.net
 From: Alice <alice@example.org>
 Subject: test message 15:53
-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
 Content-Type: multipart/signed; micalg=pgp-sha256;
  protocol="application/pgp-signature";
  boundary="------------iX39J1p7DOgblwacjo0e7jX7"