Add private_chat_portal_meta option and fix bugs

* The new option is implicitly enabled when encryption is default
* Private chat metadata is now updated after creating the room too
* The puppet metadata is updated before creating the room, to make sure their
  name is available locally

mautrix_telegram/example-config.yaml

@@ -207,6 +207,9 @@ bridge:
         # Default to encryption, force-enable encryption in all portals the bridge creates
         # This will cause the bridge bot to be in private chats for the encryption to work properly.
         default: false
+    # Whether or not to explicitly set the avatar and room name for private
+    # chat portal rooms. This will be implicitly enabled if encryption.default is true.
+    private_chat_portal_meta: false
 
     # Overrides for base power levels.
     initial_power_level_overrides: