Moved WebDAV check to client side JS

Moved WebDAV and internet checks to client side JS
    
- Added setup checks in JavaScript
- Moved isWebDAVWorking to JS using SetupChecks
- Moved internet connection checks to an ajax call that goes through the server

TODO:
- [x] Make it look less ugly
- [ ] Test with reverse proxy cases
- [x] Test with IE8+

FYI @karlitschek @DeepDiver1975 
In other files we close the session for actions that could take a long time or do timeout. - Shouldn't we do here the same?