Related to https://github.com/mautrix/discord/issues/95

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.

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

Update group_vars/matrix_servers

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

Remove old data migration tasks

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

Update roles/matrix-bridge-mautrix-discord/tasks/validate_config.yml

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

Redo bridge permissions