[WIP] Add server<->server sharing

Add support for adding public links from someone to your own owncloud

Fixes #7221
At some point we might want to let the storage classes tell us whether they allow encryption or not (`$storage->shouldEncrypt('path')`). There are some cases like for Dropbox shares where encryption should be disabled as well.

This is just something to keep in mind for later, please keep the current code.