Issue 596: cleanup_empty_projects, disable_publish_for_branches options

Normally, if a submit request is accepted and results in cleaning up the last package in a branched project, the branched project is deleted too. This is annoying if the branched project is a long-running
branch (e.g. for a developer in a small team) and has been reconfigured, for instance to turn on publishing in installations where branches do not normally have publishing.

Similarly, OBS currently disables publishing for any new branch in order to avoid using disk space and bandwidth to carry out publishing, but for small private installations, it can make sense to avoid that, and inherit the publish flags from the parent project instead.