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.

turt2live/matrix-appservice-webhooks -> redoonetworks/matrix-appservice-webhooks

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1447

Seems like there's such a variable in the
`matrix-bridge-appservice-slack` role, which is probably where it got
copied over from.

Signed-off-by: Ahmad Haghighi <haghighi@fedoraproject.org>

This is both for consistency with 93cc71cb69976c
and for making things more obvious.

In relation to #392 (Github Issue)