services/mongooseim: Move into extra subfolder.

The module options are going to get a bit larger, so let's move it to
its own directory in preparation to that.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
services/mongooseim: Add new settings submodule.

This is only a tiny fraction of all available ejabberd options, but it's
at least a start. We also use this by default for now, because the tests
currently supply their own static config files until we are done with
all options in the settings submodule.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>