[FR] Add backend-page to manage filesystem-entries

[macrozone]

• Allow admins to manage all uploaded files
• Allow admins to select a existing file instead of uploading a new file on the file/image-attribute
• Allow admins to annotate files with tags and meta-data
• For every file show where this file is used (Collection-name and id, page-url, etc.), so that an admin can verify if its save to delete or change a file

I started a PR for froala v2 that couples froala's imageManager with orionjs:filesystem. It gives a first impression how such a file-manager could work.

See #355