Support WebDAV locking

The WebDAV lock method should be supported.

Testcase:

• user a opens a word file via WebDAV
• user b can open the same file only read-only
• in OpenOLAT web GUI the file is also locked and can not be overwritten

The WebDAV Lock must be refreshed and removes itself after some time. Using the already existing Lock in the folder GUI in OpenOLAT the lock can remain valid for hours or even days. Thus, the lock is not the same but must work together transparently.