Make it possible to filter by tags with REPORT method

Enhanced the REPORT method on the Webdav endpoint and added a
"oc:favorite" filter rule. When set, it will return a flat list of
results filtered with only favorite files.

The web UI was also adjusted to use this REPORT method instead of the
private API endpoint.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>