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
We probably need to either also save the answer or make sure that the response that comes back from saving the question includes the new version of the answer so that the answer in the ember store is updated. I guess we could also just set `answer.set('isAcceptedAnswer', true);`