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.
Merge pull request #136 from coyote/unique-gallery-ids

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