Add option to disable removing avatars from Telegram ghosts

There's no way to determine whether an avatar is removed or just hidden
from some users, so avatars are not removed by default.

mautrix_telegram/puppet.py

@@ -1,5 +1,5 @@
 # mautrix-telegram - A Matrix-Telegram puppeting bridge
-# Copyright (C) 2019 Tulir Asokan
+# Copyright (C) 2020 Tulir Asokan
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU Affero General Public License as published by
@@ -300,6 +300,8 @@ class Puppet(CustomPuppetMixin):
         else:
             self.log.warning(f"Unknown user profile photo type: {type(photo)}")
             return False
+        if not photo_id and not config["bridge.allow_avatar_remove"]:
+            return False
         if self.photo_id != photo_id:
             if not photo_id:
                 self.photo_id = ""