Attachment model use in gallery

See [CYB-114](https://jira.ets.berkeley.edu/jira/browse/CYB-114) for a detailed description.

The "further back end" part of this was already merged by (Pull Request #59)[https://github.com/ets-berkeley-edu/datacultures/pull/59] -- but the api controller to serve the data to the front end was still using the old method (using the Activity model).  This completes the path from submission to the gallery.