Fixes a problem where importing would lead to this error:
    Cannot link to /matrix-postgres, as it does not belond to the default network.

Pretty much all variables live in their own `matrix_<whatever>`
prefix now and are grouped closer together in the default
variables file (`roles/matrix-server/defaults/main.yml`).

The non-working script is supposed to be fixed
by https://github.com/matrix-org/synapse/pull/2375

To have it work, we'd need an updated Docker image
of `silviof/matrix-riot-docker:latest`, which is not yet available
at the time of this commit.

Still, the previous patched synapse_port_db didn't work well either,
so it's not like we're regressing much by getting rid of it.

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.