Add sharePermissions webdav property

This property can be queries by the clients so they know the max
permissions they can use to share a file with. This will improve the UX.

The oc:permissions proptery is not enough since mountpoints have
different permissions (delete + move by default).

By making it a new property the clients can just request it. On older
servers it will just return a 404 for that property (and thus they know
they have to fall back to their hacky work arounds). But if the property
is returned the client can show proper info.

* unit tests
* intergration test