Notes:
- Add a `webdavcommon` folder for WebDAV generic resource code
- Move `davresource` to `carddaveresource` and make it use the WebDAV code
- For now it tests the CalDAV resource directly on KolabNow (to be changed)
- Only synchronization, not adding / changing / removing WebDAV collections or items (to be implemented)
- Only events are currently supported (todo, freebusy, etc. are to be implemented but should be straightforward)
- Weird warning from the `threadindexer` saying it `Found an email without messageId` when adding Calendars to the store.
Fixes T8224