[WIP] Add server<->server sharing

:construction: 

Currently, to test this you need to manually add the share to the `share_external` table, i.e:

id | remote | token | name | owner | user | mountpoint | mountpoint_hash (md5)
--- | ---- | ---- | --- | ----- | ----- | ------ | ------
1 | http://localhost/owncloud | e0aa4966221b58177f2dee596f83f998 | RemoteFoo | test | icewind | /icewind/files/RemoteFoo | 212633acfc5d2057a642a8cc01f9cf5a

Fixes #7221