RFR - Hh accept answer 2

RFR except for the following potential additions:

- [x] Sort so that `accepted_answer` appears before other answers.
- [x] Hide "Accept Answer" button for the answer that is accepted.

TODO:

- [ ] Refactor answers list template into partial (list-item template)  [NOTE:  slightly different markup for `acceptedAnswer` vs. `unselectedAnswers` in answers list template -- resolve so can use list-item template in both places.]
- [ ] Add acceptance test?
- [ ] Should update accepted answer w/o page refresh
Since this is what is run when the save fails, we should rollback any of the changes to the question.

```
question.rollback();
```