Fix typo
Move mautrix variable in `defaults/main.yml` exclusively

I've found the previous 10K default value to be way too low
on a bunch of servers I'm running, so it's now up to
100K and made configurable.

Closes Github issue #5

Closes Github issue #3

We've been using Goofys for a long time instead.

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 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)

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.

The matrix-nginx-proxy was reloaded on the 3rd day of the month (`15 4 3 * *`),
which makes no sense - it's too infrequently.

It's in line with the renewal time now (+5 minutes).

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.