Add server<->server sharing backend

apps/files_external/lib/webdav.php

apps/files_sharing/appinfo/app.php

apps/files_sharing/appinfo/database.xml

apps/files_sharing/appinfo/version

apps/files_sharing/lib/external/cache.php

apps/files_sharing/lib/external/manager.php

apps/files_sharing/lib/external/storage.php

apps/files_sharing/lib/isharedstorage.php

lib/private/connector/sabre/directory.php

lib/private/connector/sabre/objecttree.php

lib/private/files/cache/cache.php

lib/private/helper.php

lib/private/share/share.php