to prevent docker from using dir instead of file

added docs, added seperate service for database, pr comments

better path handling, fixed some mistakes

Related to #508 (Github Pull Request).

Riot unconditionally asks for a config.${document.domain}.json, c.f.
https://github.com/vector-im/riot-web/blame/develop/src/vector/getconfig.ts#L24

This is mostly here to guard against problems happening
due to server migration and doing `chown -R matrix:matrix /matrix`.

Normally, the file is owned by `1000:1000`, as expected.

If ownership changes, Dimension could still start, but it will fail the
first time it tries to write to the database. Explicitly chowning
before startup guards against this.

Related to #485 and #486 (Github Pull Requests).
Also related to ccc7aaf0ce.

Dimension runs as the `node` user in the container (`1000:1000`).
It doesn't seem like we have a way around it. Thus, its configuration
must also be readable by that user (or group, in this case).

We don't really need to fail in such a spectactular way,
but it's probably good to do. It will only happen for people
who are defining their own user/group id, which is rare.

It seems like a good idea to tell them that this doesn't work
as they expect anymore and to ask them to remove these variables,
which otherwise give them a fake sense of hope.

Related to #486 (Github Pull Request).

If one runs the playbook with `--tags=setup-all`, it would have been
fine.

But running with a specific tag (e.g. `--tags=setup-riot-web`) would
have made that initialization be skipped, and the `matrix-riot-web` role
would fail, due to missing variables.

No migration steps should be required.

This should be a no-op, no migration necessary.

Ansible will migrate the ownership of the base path and config path, but
manual intervention will be required in order to migrate the ownership
of files in those directories (i.e. dimension.db).

Stop the services:

  (local)$ ansible-playbook -i inventory/hosts setup.yml --tags=stop

Fix the permissions on the server:

  (server)# chown -Rv "{{ matrix_user_username }}:{{ matrix_user_username }}" "{{ matrix_dimension_base_path }}"

which would typically look like:

  (server)# chown -Rv matrix:matrix /matrix/dimension/

Reconfigure Dimension and start the services:

  (local)$ ansible-playbook -i inventory/hosts setup.yml --tags=setup-dimension,start