CYB-375 The date is no longer returned in the /gallery/index feed

 * adding the 3 date fields that are taken from the Canvas API return values
   for each asset type to the selected values returned

[CYB-375 The date is no longer returned in the /gallery/index feed](https://jira.ets.berkeley.edu/jira/browse/CYB-375)
@nicolaasmatthijs we've been returning epoch dates in the Rails back-end in order to avoid timezone conflicts and make it easy to parse in the front-end. It's fine to change that contract as long as we make sure it's consistent across the codebase.