The old variables still work. The global lets us avoid
auto-detection logic like we're currently doing for
`matrix_nginx_proxy_proxy_matrix_federation_api_enabled`.

In the future, we'd just be able to reference
`matrix_homeserver_federation_enabled` and know the up-to-date value
regardless of homeserver.

This was meant to serve as an intermediary for services needing to reach
the homeserver. It was used like that for a while in this
`bye-bye-nginx-proxy` branch, but was never actually public.

It has recently been superseded by homeserver-like services injecting
themselves into a new internal Traefik entrypoint
(see `matrix_playbook_internal_matrix_client_api_traefik_entrypoint_*`),
so `matrix-homeserver-proxy` is no longer necessary.

---

This is probably a good moment to share some benchmarks and reasons
for going with the internal Traefik entrypoint as opposed to this nginx
service.

1. (1400 rps) Directly to Synapse (`ab -n 1000 -c 100 http://matrix-synapse:8008/_matrix/client/versions`
2. (~900 rps) Via `matrix-homeserver-proxy` (nginx) proxying to Synapse (`ab -n 1000 -c 100 http://matrix-homeserver-proxy:8008/_matrix/client/versions`)
3. (~1200 rps) Via the new internal entrypoint of Traefik (`matrix-internal-matrix-client-api`) proxying to Synapse (`ab -n 1000 -c 100 http://matrix-traefik:8008/_matrix/client/versions`)

Besides Traefik being quicker for some reason, there are also other
benefits to not having this `matrix-homeserver-proxy` component:

- we can reuse what we have in terms of labels. Services can register a few extra labels on the new Traefik entrypoint
- we don't need services (like `matrix-media-repo`) to inject custom nginx configs into `matrix-homeserver-proxy`. They just need to register labels, like they do already.
- Traefik seems faster than nginx on this benchmark for some reason, which is a nice bonus
- no need to run one extra container (`matrix-homeserver-proxy`) and execute one extra Ansible role
- no need to maintain a setup where some people run the `matrix-homeserver-proxy` component (because they have route-stealing services like `matrix-media-repo` enabled) and others run an optimized setup without this component and everything needs to be rewired to talk to the homeserver directly. Now, everyone can go through Traefik and we can all run an identical setup

Downsides of the new Traefik entrypoint setup are that:

- all addon services that need to talk to the homeserver now depend on Traefik
- people running their own Traefik setup will be inconvenienced - they
  need to manage one additional entrypoint

We'd be adding integration with an internal Traefik entrypoint
(`matrix_playbook_internal_matrix_client_api_traefik_entrypoint`),
so renaming helps disambiguate things.

There's no need for deperecation tasks, because the old names
have only been part of this `bye-bye-nginx-proxy` branch and not used by
anyone publicly.

This also adds labels for Synapse. Support for other homeservers and
components will be added later.

This saves us one container network in the ideal case.

This is a bit of a compatibility break.
The role was defaulting the Postgres password to `some-password` and we
auto-generate it now.

However, rebuilding both Postgres and this service should unify the
database credentials and the service configs to the new value.

This service will be removed soon, but for now we need to get rid of
`matrix_docker_network` usage everywhere.

Same as 250b91a40968e, but for Slack

Postgres is not in `matrix_docker_network` anymore, so what we had
before could not possibly work anymore.

The old variables existed as well, but I inteded to use these new ones.

The user may be running Prometheus elsewhere. It doesn't need to be
getting installed using the current playbook.

Regression since ce2f541deb

This requires at least `v0.14.0-2` of the `prometheus-postgres-exporter`
Ansible role.

This requires at least `v1.7.0-2` of the `prometheus-node-exporter`
Ansible role.