cleanup_empty_projects: add new configuration option

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.

Bug: https://github.com/openSUSE/open-build-service/issues/596

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Only manually tested, so far.