WIP on commments: comments now mostly work.

Comments can now be added repeatedly, rather than only once per
answer. But the code that's supposed to be adding them to the DOM
doesn't seem to be updating. I suspect that the ng-repeat will
need to be updated by hand, but I don't yet know how to do that.
That will be for the next work-in-progress commit.
WIP on comments: comments now show up in the DOM.

Still a work-in-progress, though, because they show up without the
name of whoever wrote them, and without the current date. To add that
information, I need to return it from the DTO, which currently returns
only the ID of the new comment. I'll model the new comment DTO API on
the existing answer DTO, which returns a hash with key of "new ID" and
value of "other data".
WIP on comments: Adding comments now works right.

Adding comments now updates the DOM and pulls the current user and
date from the DTO object, just like adding answers does. The only
remaining bug is that editing an answer wipes out that answer's
comments, because part of the answer object isn't being preserved
through the edit process.