Notes:
- For calendars, only removal is implemented because:
- There is no DavCollectionCreateJob, possibly because there can't be an empty DAV collection
- DavCollectionModifyJob only allows modifying "properties", which we don't use (except for the name, if the name is considered a property)
- Currently, modifying an item with Sink overrides the one on the server, even if the store is not up-to-date