Initial solution and database update for #11

The database now contains a displayname_source field which is the
telegram user ID of the user whose point of view the displayname
is from.

Updates from the relaybot user always take precendence, but currently
the relaybot will never automatically fetch displaynames.

mautrix_telegram/abstract_user.py

@@ -42,6 +42,7 @@ class AbstractUser:
         self.client = None
         self.tgid = None
         self.mxid = None
+        self.is_relaybot = False
 
     async def _init_client(self):
         self.log.debug(f"Initializing client for {self.name}")