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/defaults/main.yml

@@ -84,9 +84,12 @@ matrix_synapse_report_stats: false
 # disabling this will decrease server load significantly.
 matrix_synapse_use_presence: true
 
-# Set autojoin room(s)
-# Users who register on this homeserver will automatically be joined to these rooms
-matrix_synapse_autojoin_rooms: []
+# Controls whether people with access to the homeserver can register by themselves.
+matrix_synapse_enable_registration: false
+
+# 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`)
+matrix_synapse_auto_join_rooms: []
 
 # Controls password-peppering for Matrix Synapse. Not to be changed after initial setup.
 matrix_synapse_password_config_pepper: ""
@@ -146,9 +149,6 @@ matrix_synapse_ext_password_provider_shared_secret_auth_download_url: "https://r
 matrix_synapse_ext_password_provider_shared_secret_auth_shared_secret: ""
 
 
-# Enable registration to new users.
-matrix_homeserver_enable_registration: false
-
 # The defaults below cause a postgres server to be configured (running within a container).
 # Using an external server is possible by tweaking all of the parameters below.
 matrix_postgres_use_external: false
@@ -271,7 +271,6 @@ matrix_riot_web_integrations_widgets_urls: "https://scalar.vector.im/api"
 matrix_riot_web_integrations_jitsi_widget_url: "https://scalar.vector.im/api/widgets/jitsi.html"
 # Riot public room directory server(s)
 matrix_riot_web_roomdir_servers: ['matrix.org']
-# Uncomment to enable default riot user bot
 matrix_riot_web_welcome_user_id: "@riot-bot:matrix.org"