CYB-233 Convert IDs to 'video-1-1' instead of '1-1'

* adds a string 'prefix' ('video-' or 'image') to the 'gallery_id' fields generated on object creation.

[CYB-233 Convert IDs to 'video-1-1' instead of '1-1'](https://jira.ets.berkeley.edu/jira/browse/CYB-233)