Item sorting feature flipper

[matthewjchandler]

We should be able to display either the updated browser with the sort options, or the existing browser from #2547.

Acceptance Criteria

• The non-sorting file browser is encapsulated into an ERB partial
• There is a new empty ERB partial for the new file browser with sorting
• When the feature is turned on the new ERB is displayed
• When the feature is turned off the old ERB is displayed
• There are test for this