Moving away from using the default bridge network to using our own.
This isolates our services from other Docker containers running
on the default network on the same host.

The benefits are that:

- isolation is a little better - we no longer share a default
bridge network with any other containers that might be running on the host

- there are no longer hard dependencies - we do service discovery
by DNS name, and not via explicit `--link` usage during container start,
so containers can start out of order and fail without bringing down others
with them
(`matrix-nginx-proxy` can continue running, even if one of the other services dies)

In the future, when other services get introduced,
the increased resilience and simplicity will help as well.

Until now, we were starting from a fresh configuration, as generated
by Synapse and manipulating it with regex and line replacements,
until we made it work.

This is more fragile and less predictable, so we're moving to a static
configuration file generated from a Jinja template.

The upside is that configuration will be stable and predictable.

The downside of this new approach is that any manual configuration changes
after the playbook is done, will be thrown away on future playbook
invocations.

There are 2 ways to work around the need for manual configuration
changes though:
- making them part of this playbook and its default template
configuration files (which benefits everyone)
- going your own way for a given host and overriding the template files
that gets used (that is, the
`matrix_synapse_template_synapse_homeserver` or
`matrix_synapse_template_synapse_log` variables)

This reverts commit 8d774db3bc.

Docker is released in the Docker CE stable repository now.

Additionally, it's version 18.03, which doesn't suffer
any of the problems we've observed with 18.05 (edge/nightly).

This playbook does not set up guest access in Synapse anyway,
so until the need comes (or someone asks for it), guest access
is removed from riot-web's UI too.

As for supporting custom URLs, this is also not something
that seems like it'd be useful to most deployments.

It's not really a requirement, as it can function without it.
Also, restarting matrix-coturn doesn't need to restart matrix-synapse.

We have 2 blockers that prevent us from adding support:

- the Docker CE repository does not publish a `docker-ce` package
in the `stable` channel. It's still in `edge`
(can be worked around by using `edge`, but we'd better not)

- Docker bind propagation has troubles on Docker CE 18.05,
which breaks matrix-synapse.service from starting, as it wants to do
a `:slave` mount. See https://github.com/moby/moby/issues/37032

Since cbee084ac19b6, this playbook supports Postgres 10.x,
but keeps existing Postgres-9.x installs on 9.x.

This playbook can now also be ran with `--tags=upgrade-postgres`
to make it upgrade from Postgres 9.x to 10.x (or other versions
in the future).

This playbook just tries to avoid trying to setup a Postgres 10
database with existing 9.x files, as that makes Postgres complain.

Due to this, existing installs (still on 9.x) are detected
and left on Postgres 9.x.
They need to be upgraded to Postgres 10.x manually.

Switching from from avhost/docker-matrix (silviof/docker-matrix)
to matrixdotorg/synapse.

The avhost/docker-matrix (silviof/docker-matrix) image used to bundle
in the coturn STUN/TURN server, so as part of the move,
we're separating this to a separately-ran service
(matrix-coturn.service, powered by instrumentisto/coturn-docker-image)

A `.log.config` file may be generated with a different
level of indentation depending on which (Docker image, etc.)
generates it.

With this patch, we tolerate different levels of indentation
(2 spaces, 4 spaces, etc.) and don't break the configuration.

Improves performance of media store operations.

Silvio announced that he's no longer maintaining his images,
so we're jumping to AVENTER-UG's fork.