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.

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

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)

Improves performance of media store operations.

As described here (
https://github.com/matrix-org/synapse/issues/2438#issuecomment-327424711
), using own SSL certificates for the federation port is more fragile,
as renewing them could cause federation outages.

The recommended setup is to use the self-signed certificates generated
by Synapse.

On the 443 port (matrix-nginx-proxy) side, we still use the Let's Encrypt
certificates, which ensures API consumers work without having to trust
"our own CA".

Having done this, we also don't need to ever restart Synapse anymore,
as no new SSL certificates need to be applied there.

It's just matrix-nginx-proxy that needs to be restarted, and it doesn't
even need a full restart as an "nginx reload" does the job of swithing
to the new SSL certificates.

Moving keeps everything in the /matrix directory, so that we
wouldn't contaminate anything else on the system or risk
clashing with something else.

Also retrieving certificates separately for the Riot and Matrix domains,
which should help in multiple ways:

- allows them to be very different (completely separate base domain..)

- allows for Riot to be disabled for the playbook some time later
  and still have the code not break

The goal is to allow these to be on separate partitions
(including remote ones in the future).

Because the `silviof/docker-matrix` image chowns
everything to MATRIX_UID:MATRIX_GID on startup,
we definitely don't want to include `media_store` in it.
If it's on a remote FS, it would cause a slow startup.

Also, adding some safety checks to the "import media store"
task, after passing a wrong path to it on multiple occassions and
wondering what's wrong.

Also, making logging configurable. The default of keeping 10x100MB
log files is likely excessive and people may want to change that.