Add alias support 2 (#2297)

fix  #2073
fix #2292 (I think)

- Messages can be assigned to any chat by the References and In-Reply-To, also to 1:1 chats; this has higher priority than the group id because with ad-hoc groups, it can happen that two devices have different group ids for the same conversation thread.
- If `From` is not in the chat (we call this "shadow sender"), `OverrideSenderDisplayname` is set. This communicates to the UI that:
  - A `~`should be added in front of the sender's displayname.
  - Also in 1:1 chats, the sender's displayname and avatar is shown, as if this was a group.

  The "Unknown sender for this chat" messages are completely removed for unprotected groups.

For protected chats, everything stays as it was before.

POSTPONED:

- Maybe (if it turns out to be still necessary):
  - The ad-hoc group id is computed by the the References, instead of the member list, as it is currently done
  - How do we prevent that the message can't be assigned to the correct chat as the parent message was deleted?

CHANGELOG.md

@@ -42,6 +42,8 @@
 - improve compatibility with providers changing the Message-ID
   (as Outlook.com) #2250 #2265
 
+- correctly show emails that were sent to an alias and then bounced 
+
 - implement Consistent Color Generation (XEP-0392),
   that results in contact colors be be changed #2228 #2229 #2239