Files sharing ported to Share API 2

The port of the files_sharing app to the Share API 2. This is a branch of https://github.com/owncloud/core/pull/3969 so you can just checkout this branch to test it.

Things that don't work:
- Share status icons
- Reshare status
- Expiration time
- Public links

The old share ajax file was hacked a bit to get it working, that's why some of the above things are broken. The new controller has already been written and is much cleaner. It's just the UI changes that are slowing things down now.

Classes that you should look at to understand the Share API changes:
- https://github.com/owncloud/core/blob/files_sharing-ported/apps/files_sharing/lib/share/filesharebackend.php
- https://github.com/owncloud/core/blob/files_sharing-ported/apps/files_sharing/lib/share/foldersharebackend.php
- https://github.com/owncloud/core/blob/files_sharing-ported/apps/files_sharing/lib/share/filesharefactory.php
- https://github.com/owncloud/core/blob/files_sharing-ported/apps/files_sharing/lib/share/filesharefetcher.php
- https://github.com/owncloud/core/blob/files_sharing-ported/apps/files_sharing/appinfo/app.php

@karlitschek @DeepDiver1975