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)
I'm all for using epoch dates. The issue here is that the API response on /gallery/index looks like this:

```
{
  'files': [
    {
      'type': 'image',
      'likes': 0,
      'dislikes': 0,
      'liked': null,
      'assignment_id: '2'
      'submission_id': '9',
      'attachment_id': '8',
      'image_url': 'http://localhost:3100/fi...espGmmaUO2RPMjFTjSG01wi',
      'author': 'Nicolaas Matthijs',
      'id': 'image-7-2',
      'canvas_user_id': '3',
      'content_type': 'image/jpeg',
      'comment_count': 2,
      'views': 22,
      'int8': 1418685278128 <--
    }, ...
  ]
}
```

instead of

```
{
  'files': [
    {
      'type': 'image',
      'likes': 0,
      'dislikes': 0,
      'liked': null,
      'assignment_id: '2'
      'submission_id': '9',
      'attachment_id': '8',
      'image_url': 'http://localhost:3100/fi...espGmmaUO2RPMjFTjSG01wi',
      'author': 'Nicolaas Matthijs',
      'id': 'image-7-2',
      'canvas_user_id': '3',
      'content_type': 'image/jpeg',
      'comment_count': 2,
      'views': 22,
      'date': 1418685278128 <--
    }, ...
  ]
}
```