Make roles more independent of one another

With this change, the following roles are now only dependent
on the minimal `matrix-base` role:
- `matrix-corporal`
- `matrix-coturn`
- `matrix-mailer`
- `matrix-mxisd`
- `matrix-postgres`
- `matrix-riot-web`
- `matrix-synapse`

The `matrix-nginx-proxy` role still does too much and remains
dependent on the others.

Wiring up the various (now-independent) roles happens
via a glue variables file (`group_vars/matrix-servers`).
It's triggered for all hosts in the `matrix-servers` group.

According to Ansible's rules of priority, we have the following
chain of inclusion/overriding now:
- role defaults (mostly empty or good for independent usage)
- playbook glue variables (`group_vars/matrix-servers`)
- inventory host variables (`inventory/host_vars/matrix.<your-domain>`)

All roles default to enabling their main component
(e.g. `matrix_mxisd_enabled: true`, `matrix_riot_web_enabled: true`).
Reasoning: if a role is included in a playbook (especially separately,
in another playbook), it should "work" by default.

Our playbook disables some of those if they are not generally useful
(e.g. `matrix_corporal_enabled: false`).

roles/matrix-riot-web/templates/config.json.j2

@@ -1,13 +1,13 @@
 {
-    "default_hs_url": "{{ matrix_homeserver_url }}",
-    "default_is_url": "{{ matrix_identity_server_url }}",
-    "disable_custom_urls": {{ matrix_riot_web_disable_custom_urls|lower }},
-    "disable_guests": {{ matrix_riot_web_disable_guests|lower }},
+    "default_hs_url": {{ matrix_riot_web_default_hs_url|to_json }},
+    "default_is_url": {{ matrix_riot_web_default_is_url|to_json }},
+    "disable_custom_urls": {{ matrix_riot_web_disable_custom_urls|to_json }},
+    "disable_guests": {{ matrix_riot_web_disable_guests|to_json }},
     "brand": "Riot",
-    "integrations_ui_url": "{{ matrix_riot_web_integrations_ui_url }}",
-    "integrations_rest_url": "{{ matrix_riot_web_integrations_rest_url }}",
+    "integrations_ui_url": {{ matrix_riot_web_integrations_ui_url|to_json }},
+    "integrations_rest_url": {{ matrix_riot_web_integrations_rest_url|to_json }},
     "integrations_widgets_urls": {{ matrix_riot_web_integrations_widgets_urls|to_json }},
-    "integrations_jitsi_widget_url": "{{ matrix_riot_web_integrations_jitsi_widget_url }}",
+    "integrations_jitsi_widget_url": {{ matrix_riot_web_integrations_jitsi_widget_url|to_json }},
     "bug_report_endpoint_url": "https://riot.im/bugreports/submit",
     "enableLabs": true,
     "roomDirectory": {