[dist] activate webui test suite again
[backend] support 'productrepositories' view
[api] try to avoid emails getting lost because of conflicting settings

If you're source_maintainer (with default false) and target_maintainer (with
default true), you get in doubt no email. If or not depends on the ruby sort
function ;(
[ci] require xml::simple for travis
[webui] fix repository_state and rebuild_time