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
I don't think so, but let me talk it through.

Putting an index on would make the following query faster, right?

```ruby
answer = # some answer
question = Question.find_by(accepted_answer: answer)
```
In reality, though, we'd probably do
```ruby
question = answer.question
```

which shouldn't really need an index, right, since you're just using `question_id` to find `Question` directly?