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-postgres/tasks/upgrade_postgres.yml

@@ -22,8 +22,8 @@
 
 - name: Fail, if trying to upgrade external Postgres database
   fail:
-    msg: "Your configuration indicates that you're using an external Postgres database. Refusing to try and upgrade that."
-  when: "matrix_postgres_use_external"
+    msg: "Your configuration indicates that you're not using Postgres from this role. There is nothing to upgrade."
+  when: "not matrix_postgres_enabled"
 
 - name: Check Postgres auto-upgrade backup data directory
   stat: