Fixes #255 (Github Issue).

Fixes #71 (Github Issue).

Continuation of #234 (Github Pull Request).

I had unintentionally updated the documentation for the feature,
saying the page is available at `https://matrix.DOMAIN/nginx_status`.

Looks like it wasn't the case, going against my expectations.

I'm correcting this with this patch.
The status page is being made available on both HTTP and HTTPS.
Serving over HTTP is likely necessary for services like
Longview
(https://www.linode.com/docs/platform/longview/longview-app-for-nginx/)

It's been pointed out that DEBUG logs could contain sensitive
information (access tokens, etc.), which makes them unsuitable
for sharing with others. INFO should be enough.

Also discussed previously in #213 (Github Pull Request).

shared-secret-auth and rest-auth logging is still at `INFO`
intentionally, as user login events seem more important to keep.
Those modules typically don't spam as much.

Fixes #216 (Github Issue).

We do restart Synapse explicitly, but some other services
(bridges, matrix-corporal, ..) may not restart sometimes.

It's best to restart all services explicitly.

Somewhat related to #213 (Github Pull Request).

We've been moving in the opposite direction for quite a long time.
All services should just leave logging to systemd's journald.

This is a new feature of Synapse v1.1.0.

Discussed in #145 (Github Pull Request).

Well, `config.yaml` has been playbook-managed for a long time.
It's now extended to match the default sample config of the Discord
bridge.

With this patch, we also make `registration.yaml` playbook-managed,
which leads us to consistency with all other bridges.

Along with that, we introduce `./config` and `./data` separation,
like we do for the other bridges.

With most people on Synapse v0.99+ and Synapse v1.0 now available,
we should no longer try to be backward compatible with Synapse 0.34,
because this just complicates the instructions for no good reason.

Changes to the original are:
- it tells people to stop and disable services, so that:
   - services won't be running while you are copying files
   - services won't accidentally start again later
- it does the file-copying in 1 step
- it does copying before running `--tags=setup-all`, so that existing files (SSL certificates, etc.) can be reused. Otherwise, the playbook starts from a blank slate, retrieves them anew, generates new signing keys anew, etc. Only to have those replaced by your own old backup later.
- it mentions DNS changes
- combines `--tags=setup-all,start` into a single step, thanks to the files being already copied

This doesn't replace all usage of `-v`, but it's a start.

People sometimes troubleshoot by deleting files (especially bridge
config files). Restarting Synapse with a missing registration.yaml file
for a given bridge, causes the `-v
/something/registration.yaml:/something/registration.yaml:ro` option
to force-create `/something/registration.yaml` as a directory.

When a path that's provided to the `-v` option is missing, Docker
auto-creates that path as a directory.
This causes more breakage and confusion later on.

We'd rather fail, instead of magically creating directories.
Using `--mount`, instead of `-v` is the solution to this.

From Docker's documentation:

> When you use --mount with type=bind, the host-path must refer to an existing path on the host.
> The path will not be created for you and the service will fail with an error if the path does not exist.