Collection support for filesystem items

Summary:
This patch adds support for collections and relations to group items
into collections.

This patch also:

• Replaces the FilesController with FsController
• Grants the companion app access to the files api (via scope)
• Allows muliple files with the same name
• Allows the client to set the mimetype

Fixed filestorage tests

Support for collections

Create collections with some metadata

UI support for collections

Differential Revision: https://git.kolab.org/D4190