Soft delete issuables

Merged Zeger-Jan van de Weg requested to merge 2489-soft-delete-issues into master

Fixes #2489 (closed)

What still needs to happen: research on the indexes, the gem suggests a lot of changes though this is probably a good idea to discuss and I'm unsure on the impact of an omnibus upgrade as I suspect creating about 10 new indexes has a large impact on the downtime.

TODO:

  • Also group owners can soft delete
  • Button should be hidden