Reportback Files API endpoint

Refs #3285

Creates API endpoint for getting Reportback Files, which will be used in the new Reportback Gallery to dynamically load more Reportback Files as users scroll through the gallery.

Will update the wiki once merged.

Example URLs:

http://dev.dosomething.org:8888/api/v1/reportback_files.json?parameters[nid]=362

http://dev.dosomething.org:8888/api/v1/reportback_files.json?parameters[nid]=362&page=10&pagesize=2