No migration steps should be required.

As suggested in #63 (Github issue), splitting the
playbook's logic into multiple roles will be beneficial for
maintainability.

This patch realizes this split. Still, some components
affect others, so the roles are not really independent of one
another. For example:
- disabling mxisd (`matrix_mxisd_enabled: false`), causes Synapse
and riot-web to reconfigure themselves with other (public)
Identity servers.

- enabling matrix-corporal (`matrix_corporal_enabled: true`) affects
how reverse-proxying (by `matrix-nginx-proxy`) is done, in order to
put matrix-corporal's gateway server in front of Synapse

We may be able to move away from such dependencies in the future,
at the expense of a more complicated manual configuration, but
it's probably not worth sacrificing the convenience we have now.

As part of this work, the way we do "start components" has been
redone now to use a loop, as suggested in #65 (Github issue).
This should make restarting faster and more reliable.

fixes #62

This is a simplification and a way to make it consistent with
how we do Postgres imports (see 6d89319822), using
files coming from the server, not from the local machine.

By encouraging people NOT to use local files,
we potentially avoid problems such as #34 (Github issue),
where people would download `media_store` to their Mac's filesystem
and case-sensitivity issues will actually corrupt it.

By not encouraging local files usage, it's less likely that
people would copy (huge) directories to their local machine like that.

Previously, it was more necessary to have it
(because we had a dependency between matrix-synapse and matrix-nginx-proxy)..
But nowadays, it can be removed without negative side effects.

Restarting matrix-nginx-proxy is especially bad when the proxy is not installed at all.

Improves performance of media store operations.

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.