display and edit hits and packs in frontend

## Summary * add frontend to display and edit hits: - all users should be able to see all packs and all hits in packs as well as hits on individual pages - admins should be able to edit hits, as well as add new hits and packs - newly added hits, or hits which got their link or playback offset changed need to re-download immediately - hits should be exportable, either individually, by pack or all hits as one, so that they can be merged back into the codebase ### Motivation Now that the database supports storing hits and packs, we're finally able to edit hits and packs in real-time. This issue should be the basis for implementing a first iteration of how this could look like, beginning with displaying hits and packs and filtering it, and later adding editing to the featureset.