Allow configuring synapse `auto_join_mxid_localpart`

`auto_join_mxid_localpart` defines the local part of the user id which is used to create auto-join rooms. The variable needs to be set to invite new users to any auto-join rooms which are set to invite-only.

roles/custom/matrix-synapse/defaults/main.yml

@@ -537,12 +537,18 @@ matrix_synapse_account_threepid_delegates_msisdn: ''
 
 # Users who register on this homeserver will automatically be joined to these rooms.
 # Rooms are to be specified using addresses (e.g. `#address:example.com`)
+# If any auto-join rooms are invite-only, you need to define `matrix_synapse_auto_join_mxid_localpart`.
 matrix_synapse_auto_join_rooms: []
 
 # Controls whether auto-join rooms (`matrix_synapse_auto_join_rooms`) are to be created
 # automatically if they don't already exist.
 matrix_synapse_autocreate_auto_join_rooms: true
 
+# The local part of the user id which is used to create auto-join rooms if `matrix_synapse_autocreate_auto_join_rooms` is true.
+# Defaults to the initial user account that registers.
+# The user id is also used to invite new users to any auto-join rooms which are set to invite-only.
+matrix_synapse_auto_join_mxid_localpart: ''
+
 # Controls whether room invites will be accepted on behalf of users.
 # See: https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html#auto-accept-invites
 # This should not be used together with the `synapse_auto_accept_invite` module (see `matrix_synapse_ext_synapse_auto_accept_invite_enabled`).

roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -1541,7 +1541,9 @@ autocreate_auto_join_rooms: {{ matrix_synapse_autocreate_auto_join_rooms|to_json
 # Note that, if the room already exists, this user must be joined and
 # have the appropriate permissions to invite new members.
 #
-#auto_join_mxid_localpart: system
+{% if matrix_synapse_auto_join_mxid_localpart %}
+auto_join_mxid_localpart: {{ matrix_synapse_auto_join_mxid_localpart }}
+{% endif %}
 
 # When auto_join_rooms is specified, setting this flag to false prevents
 # guest accounts from being automatically joined to the rooms.