Files plugin: enable access control (#409).

File operations are matter of access control:
    - "Upload File"   is enabled only for users who have CREATE permission for the "File"   type.
    - "Create Folder" is enabled only for users who have CREATE permission for the "Folder" type.

Thus, by default guests can neither upload files nor create folders.
For logged in users file operations can be access controlled by workspace assignments: to disable file operations for certain users you can assign the users to a workspace the File/Folder types are *not* assigned to.

See ticket 409.