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/config.py

@@ -83,6 +83,7 @@ class Config(BaseBridgeConfig):
 
         copy("bridge.displayname_preference")
         copy("bridge.displayname_max_length")
+        copy("bridge.allow_avatar_remove")
 
         copy("bridge.max_initial_member_sync")
         copy("bridge.sync_channel_members")