CYB-230 Refactor the Comments API

* migration is required (new fields,  renamed field, field type changed)
* model changes to create associations so values can be retrieved in a nested manner
* association between images, videos, and the comments is now on a new field on each, 'gallery_id', which is a post-create field
* code is fairly fragile

* comments no longer nested
Merge pull request #123 from coyote/comments_api_refactor

CYB-230 Refactor the Comments API
CYB-235 - Comments: hook the front-end up to the back-end

https://jira.ets.berkeley.edu/jira/browse/CYB-235
Merge pull request #124 from christianv/CYB-235

CYB-235 - Comments: hook the front-end up to the back-end