Add Issue tracker for hits

[Timtam]

Summary

Sometimes hits become unavailable since their links get invalidated. Sometimes time offsets or years/artists/etc are wrong and need fixing. This issue introduces an issue tracker for hits.

• hits should validate wether their link / id is still available, and if it isn't, they should be marked as "needs fixing"
  • hits can be marked as "needs fixing" with a message on what needs to be done
  • maybe do that when they get selected and played in a game?
  • in addition to that, users should be able to report when hits need adjustments
  • users should not be required to login to do that, the form should however be protected by captcha
  • admins should be able to see the list of issues and delete them / mark them as fixed
  • introduce a filter for hits with issues