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-mailer/defaults/main.yml

@@ -1,6 +1,3 @@
-# By default, this playbook sets up a postfix mailer server (running in a container).
-# This is so that Matrix Synapse can send email reminders for unread messages.
-# Other services (like mxisd), however, also use that mailer to send emails through it.
 matrix_mailer_enabled: true
 
 matrix_mailer_base_path: "{{ matrix_base_data_path }}/mailer"

roles/matrix-mailer/tasks/main.yml

@@ -5,5 +5,5 @@
 - import_tasks: "{{ role_path }}/tasks/setup_mailer.yml"
   when: run_setup
   tags:
-    - setup-mailer
-    - setup-all
+    - setup-all
+    - setup-mailer