Add new Option to Show Answer Dropdown.

In Studio: Introduces a new option, `after some number of attempts`
and a new entry box for specifying the number of attempts.
This allows course creators to specify that a given question's
answer is only viewable, i.e its show answer button is visible,
after the learner has attempted answering the question -
by hitting the submit button - a given number of times. Included
in this commit are unit tests for the new feature.