Fix multi-choice answer issue & extract answer bodies to partials

Multi-choice answer responses were failing to save because we were trying to pass an id instead of a multi_choice_option_id.

We created a new branch to fix the issue, then followed the same PR refactor requests from our previous branch.

Finally, we extracted answers to partials, removed the validation on body in models and removed the body column from multi_choice_answers.

This is doing exactly the same thing as `has_many :multi_choice_options`, I think. What if you do `.multi_choice_options` where you're doing `.options`, or just redefine this method:

```ruby
def options
  multi_choice_options
end
```