Display comments on items in editor; include set comments

Improve back-end access to user-generated item comments. New functionality should include:

1. Display of user-generated comments in editor user interface. Comments should be a simple list with display of text content and option to delete.

2. Comments made on items in the context of a set should be included in the display with a reference to the set context.

3. User-generated content UI should be expanded to allow display of comments by specific users; list should include comments made in the context of a specific set. References should be provided to both the item commented upon as well as the set.