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/1931

`|to_json` already adds the surrounding double-quotes for the string, so
adding them explicitly means we've got a double-quotes problem.

* Implement MSC 1929 Homeserver Admin Contact
Fixes #1612

For details to the proposed (not accepted yet) MSC, see:
https://github.com/matrix-org/matrix-spec-proposals/blob/hs/proposal-admin-contact-1/proposals/1929-admin-contact.md

* Implement feedback from PR #1931

* Implement feedback from PR #1931

* Fix mixed indentation