[WIP] Moved WebDAV check to client side JS

:no_entry_sign: NOMERGE :construction: WIP

- Added post setup checks in JavaScript
- Added SetupChecks class that makes it possible to run multiple checks
  in parallel (currently only one implemented)
- Moved isWebDAVWorking to JS using SetupChecks

TODO:
- [x] Make it look less ugly
- [ ] Test with reverse proxy cases
- [ ] Test with IE8+

FYI @karlitschek @DeepDiver1975