CYB-271 Support for number of views of Gallery items

Support for tracking views
  * new model, controller, route for views
  * gallery controller will return 'views' count(actually implemented in lib/array_refinement and
     in the attachment and media_url models)

[CYB-271 Support for number of views of Gallery items](https://jira.ets.berkeley.edu/jira/browse/CYB-271)
@coyote does this mean that it's only included for images? Shouldn't we also have this for videos?