* refactor: cleanup send_handshake_msg()
- rename to send_alice_handshake_msg() as used by Alice only
- remove dead code from Bob
(Bob's code is at BobState::send_handshake_message() since some time)
- take a contact_id and not a chat_id;
this makes things less confusing when
info-messages are put to the final group chat
* always directly return chat-id from dc_join_securejoin()
* take care not to create a group twice
* adapt documentation
* add info-msg on group invites; add inviter directly after creation
* document existing 'joinqr' command in repl tool
* do not create empty one-to-one chats for group-joins
* refactor: cleanup fingerprint_equals_sender()
- the function takes a contact_id directly now.
before it consumes the first contact of a one-to-one chat -
which may be easily confused with the group-chat in creation.
moreover, the conversion contact_id -> chat_id -> contact_id
is unneeded overhead.
* show info-messages in destination chat for alice
* fingerprint_equals_sender() returns Err on database failure
* tweak documentation
* clarify what an 'unfinished tasks' task is.
* add regression test for create_for_contact_with_blocked()
* rename Blocked::Manually to better fitting Blocked::Yes
* tweak test_secure_join() and make sure, Alice and Bob have only on chat after a group-join
3b7b8ea0f1