Fix a few minor things for consistency

Trying to:

- stay closer to naming in Synapse (autojoin -> auto_join)

- not create new variable namespaces (`matrix_homeserver_`),
when existing ones (`matrix_synapse_`) are more suitable

- allow `null` (`~`) values for `matrix_riot_web_welcome_user_id`

- render things like `auto_join_rooms` in `homeserver.yaml` more prettily

- fix breakage in `config.json` where `matrix_riot_web_roomdir_servers`
was rendered as YAML and not as JSON

- simplify code (especially in riot-web's `config.json`), which used
`if` statements that could have been omitted

- avoid changing comments in `homeserver.yaml` which are not ours,
so that we can keep closer to the configuration file generated by upstream

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

@@ -432,7 +432,7 @@ turn_allow_guests: False
 ## Registration ##
 
 # Enable registration for new users.
-enable_registration: {{ matrix_homeserver_enable_registration|capitalize }}
+enable_registration: {{ matrix_synapse_enable_registration }}
 
 # The user must provide all of the below types of 3PID when registering.
 #
@@ -474,11 +474,13 @@ trusted_third_party_id_servers:
     - {{ server }}
 {% endfor %}
 
-# Users who register on this homeserver will automatically be joined to these rooms
-{% if matrix_synapse_autojoin_rooms|length >0 %}
+# Users who register on this homeserver will automatically be joined
+# to these rooms
 #auto_join_rooms:
 #    - "#example:example.com"
-auto_join_rooms: {{ matrix_synapse_autojoin_rooms|to_yaml }}
+{% if matrix_synapse_auto_join_rooms|length > 0 %}
+auto_join_rooms:
+{{ matrix_synapse_auto_join_rooms|to_nice_yaml }}
 {% endif %}
 
 ## Metrics ###