Add Answer#has_been_answered?

- And move Answer#has_not_been_answered? out of the `private` block.
Add Answer#clear_content
Add Response#update_answers.
Response#update_answers returns `true` if no params are passed in.
When updating answers based on params, use `try` so it doesn't bomb if the params aren't passed in.
Use `detect` instead of `find_by_id` so that the errors are attached on each answer.

- Also wait till all answers are updated before raising an `ActiveRecord::Rollback` so that errors can be attached on all answers that have an error.
Response#update_answers returns true if all answers are saved successfully.