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 (e.g., no testing for failure to save)
* comments no longer nested

[CYB-230 Refactor the Comments API](https://jira.ets.berkeley.edu/jira/browse/CYB-230)