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/bot.py

@@ -41,6 +41,7 @@ class Bot(AbstractUser):
         self.token = token
         self.whitelisted = True
         self.username = None
+        self.is_relaybot = True
         self.chats = {chat.id: chat.type for chat in BotChat.query.all()}
         self.tg_whitelist = []
         self.whitelist_group_admins = config["bridge.relaybot.whitelist_group_admins"] or False