Add gvaweb and gvaldap containers to docker-compose.yml. Unify most of
Dockerfile with gvaweb and gvaldap. Add empty directories for mounting
asset and media files into bind mounted docker volumes. Run application
as separate system user.
This commit is a rough port to Django 2.1, Python 3 and a Docker based local
development setup. Tests fail/error but migrations and the web frontend are
already runnable. Task queue functionality is untested and translations seem to
have trouble.
