Whenever the server returns true for the can-assign Webdav property of
a system tag, it means the current user is allowed to assign,
regardless of the value of user-assignable.
This commit brings the proper logic to the web UI to make it possible
for users to assign when they have the permission.
add checkbox to toggle show/hide hidden files
persist show hidden setting
fix settings menu layout
test ApiController::showHiddenFiles
don't show hidden files by default
Store config in Backbone model and inject it into FileList
Filter files only temporarily when rending the file list
Fix file rename validation
For admins: display the namespace behind the tag name.
For users: no namespace, don't display non-assignable tags in the
dropdown, display already assigned non-assignable tags with a different
style