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.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1939

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1682

Previously, when matrix-postgres was disabled, we were setting
`matrix_mautrix_twitter_database_engine` to an invalid empty value.

Now, we always hardcode `matrix_mautrix_twitter_database_engine: postgres`,
but set/unset the database hostname and password values instead.

Remove references to sqlite since we should just use postgres from the
outset.

find roles/matrix-bridge-mautrix-twitter -type f -exec sed -i \
  -e 's/facebook/twitter/g' \
  -e 's/Facebook/Twitter/g' \
  -e 's/FB/Twitter/g' \
  -e 's/fb/twitter/g' {} +