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-element/tasks/prepare_themes.yml

@@ -0,0 +1,47 @@
+---
+
+#
+# Tasks related to setting up Element themes
+#
+
+- when: matrix_client_element_themes_enabled | bool
+  run_once: true
+  delegate_to: 127.0.0.1
+  become: false
+  block:
+  - name: Ensure Element themes repository is pulled
+    ansible.builtin.git:
+      repo: "{{ matrix_client_element_themes_repository_url }}"
+      version: "{{ matrix_client_element_themes_repository_version }}"
+      dest: "{{ role_path }}/files/scratchpad/themes"
+
+  - name: Find all Element theme files
+    ansible.builtin.find:
+      paths: "{{ role_path }}/files/scratchpad/themes"
+      patterns: "*.json"
+      recurse: true
+    register: matrix_client_element_theme_file_list
+
+  - name: Read Element theme
+    ansible.builtin.slurp:
+      path: "{{ item.path }}"
+    register: "matrix_client_element_theme_file_contents"
+    with_items: "{{ matrix_client_element_theme_file_list.files }}"
+
+  - name: Load Element theme
+    ansible.builtin.set_fact:
+      matrix_client_element_settingDefaults_custom_themes: "{{ matrix_client_element_settingDefaults_custom_themes + [item['content'] | b64decode | from_json] }}"  # noqa var-naming
+    with_items: "{{ matrix_client_element_theme_file_contents.results }}"
+
+#
+# Tasks related to getting rid of Element themes (if it was previously enabled)
+#
+
+- name: Ensure Element themes repository is removed
+  ansible.builtin.file:
+    path: "{{ role_path }}/files/scratchpad/themes"
+    state: absent
+  run_once: true
+  delegate_to: 127.0.0.1
+  become: false
+  when: "not matrix_client_element_themes_enabled | bool"