Soft delete issuables
Fixes#2489
What still needs to happen: research on the indexes, the gem suggests a [lot of changes](https://github.com/rubysherpas/paranoia#about-indexes) 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:
- [x] Also group owners can ***soft*** delete
- [x] Button should be hidden
See merge request !2982
Improve performance of viewing individual issues
This MR does two things:
1. `Issue#related_branches` no longer performs Git operations that aren't needed
2. The output of `Repository#exists?` is now cached and flushed properly
Combined these two changes should further cut down the amount of Git operations performed when viewing individual issues (and possibly other pages).
See merge request !3296
Douwe MaanandRémy Coutable
Robert SpeicherandRémy Coutable
Francesco Levorato
Stan Hu