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/riot-web/config.json.j2

@@ -11,11 +11,7 @@
     "bug_report_endpoint_url": "https://riot.im/bugreports/submit",
     "enableLabs": true,
     "roomDirectory": {
-        "servers": {{ matrix_riot_web_roomdir_servers|to_nice_yaml }}
-{% if matrix_riot_web_welcome_user_id %}
+        "servers": {{ matrix_riot_web_roomdir_servers|to_json }}
     },
-    "welcomeUserId": "{{ matrix_riot_web_welcome_user_id }}"
-{% else %}
-    }
-{% endif %}
+    "welcomeUserId": {{ matrix_riot_web_welcome_user_id|to_json }}
 }