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.

docs/configuring-playbook-client-element.md

@@ -17,7 +17,7 @@ matrix_client_element_enabled: false
 
 The playbook provides some customization variables you could use to change Element's settings.
 
-Their defaults are defined in [`roles/matrix-client-element/defaults/main.yml`](../roles/matrix-client-element/defaults/main.yml) and they ultimately end up in the generated `/matrix/element/config.json` file (on the server). This file is generated from the [`roles/matrix-client-element/templates/config.json.j2`](../roles/matrix-client-element/templates/config.json.j2) template.
+Their defaults are defined in [`roles/custom/matrix-client-element/defaults/main.yml`](../roles/custom/matrix-client-element/defaults/main.yml) and they ultimately end up in the generated `/matrix/element/config.json` file (on the server). This file is generated from the [`roles/custom/matrix-client-element/templates/config.json.j2`](../roles/custom/matrix-client-element/templates/config.json.j2) template.
 
 **If there's an existing variable** which controls a setting you wish to change, you can simply define that variable in your configuration file (`inventory/host_vars/matrix.<your-domain>/vars.yml`) and [re-run the playbook](installing.md) to apply the changes.
 
@@ -25,9 +25,9 @@ Alternatively, **if there is no pre-defined variable** for an Element setting yo
 
 - you can either **request a variable to be created** (or you can submit such a contribution yourself). Keep in mind that it's **probably not a good idea** to create variables for each one of Element's various settings that rarely get used.
 
-- or, you can **extend and override the default configuration** ([`config.json.j2`](../roles/matrix-client-element/templates/config.json.j2)) by making use of the `matrix_client_element_configuration_extension_json_` variable. You can find information about this in [`roles/matrix-client-element/defaults/main.yml`](../roles/matrix-client-element/defaults/main.yml).
+- or, you can **extend and override the default configuration** ([`config.json.j2`](../roles/custom/matrix-client-element/templates/config.json.j2)) by making use of the `matrix_client_element_configuration_extension_json_` variable. You can find information about this in [`roles/custom/matrix-client-element/defaults/main.yml`](../roles/custom/matrix-client-element/defaults/main.yml).
 
-- or, if extending the configuration is still not powerful enough for your needs, you can **override the configuration completely** using `matrix_client_element_configuration_default` (or `matrix_client_element_configuration`). You can find information about this in [`roles/matrix-client-element/defaults/main.yml`](../roles/matrix-client-element/defaults/main.yml).
+- or, if extending the configuration is still not powerful enough for your needs, you can **override the configuration completely** using `matrix_client_element_configuration_default` (or `matrix_client_element_configuration`). You can find information about this in [`roles/custom/matrix-client-element/defaults/main.yml`](../roles/custom/matrix-client-element/defaults/main.yml).
 
 
 ## Themes