ACL management for DAV folders

Summary:
Implement DAV folder sharing based on draft-pot-webdav-resource-sharing standard

We keep the DAV ACL standard implementation as an option, but this standard
does not cover the folder discovery, so we'll not use it with Kolab setups.

Reviewers: Roundcube Kolab Plugins Developers

Subscribers: Roundcube Kolab Plugins Developers

Differential Revision: https://git.kolab.org/D4668