Move roles/matrix* to roles/custom/matrix*

This paves the way for installing other roles into `roles/galaxy` using `ansible-galaxy`,
similar to how it's done in:

- https://github.com/spantaleev/gitea-docker-ansible-deploy
- https://github.com/spantaleev/nextcloud-docker-ansible-deploy

In the near future, we'll be removing a lot of the shared role code from here
and using upstream roles for it. Some of the core `matrix-*` roles have
already been extracted out into other reusable roles:

- https://github.com/devture/com.devture.ansible.role.postgres
- https://github.com/devture/com.devture.ansible.role.systemd_docker_base
- https://github.com/devture/com.devture.ansible.role.timesync
- https://github.com/devture/com.devture.ansible.role.vars_preserver
- https://github.com/devture/com.devture.ansible.role.playbook_runtime_messages
- https://github.com/devture/com.devture.ansible.role.playbook_help

We just need to migrate to those.

roles/custom/matrix-client-hydrogen/templates/config.json.j2

@@ -0,0 +1,12 @@
+{
+	"push": {{ matrix_client_hydrogen_push | to_json }},
+	"defaultHomeServer": {{ matrix_client_hydrogen_default_hs_url | string | to_json }},
+	"bugReportEndpointUrl": {{ matrix_client_hydrogen_bugReportEndpointUrl | to_json }},
+	"themeManifests": [
+		"assets/theme-Element.json"
+	],
+	"defaultTheme": {
+		"light": "element-light",
+		"dark": "element-dark"
+	}
+}