Add 'None of the above' and 'Show 3/4/5 answers' as Multiple Choice options

Summary:
Adds the functionality to write several answers but only show a subset of them and (optionally)a "None of the above" (NOTA) answer. Without the NOTA, it will guarantee that one of the subset is correct. (With the NOTA enabled, NOTA may be right and the rest wrong). There is a safeguard for if they choose NOTA and another option, which returns invalid (ungraded) because it will never be correct. Should work for multi-select and regular radio.

Known Issues:
- Correctly selecting NOTA when multiple answers are correct shows all of them, with only one checked (either all should be checked, or none should show for multiple-select)
- 'None of the above' doesn't have a clue (Maybe NOTA should appear as an option in the editor?). Alternatively, they could write in NOTA themselves, and we just separately guarantee that it will end up at the end. I like that better.

Test Plan:
- Open radio, type in several choices, check that everything works as expected:
-- None selected shows up as invalid / empty
-- Without multiple select:
--- Correct answer shows up as correct
--- Incorrect answers show up as incorrect
-- With multiple select
--- Selecting single correct answer among >= 2 correct shows up as incorrect
--- Selecting all correct answers show up as correct
--- Selecting any combination including incorrect answers is incorrect

(1) Choose randomize and limit 3/4/5, check that it works
-- A correct answer should always be within the displayed choices
-- Test by trying switching the correct answer to not be included
(2) Choose randomize and NOTA, check that it works
-- NOTA will be correct only if no answers are selected as correct
-- NOTA should not disappear
-- It should still complain if no answers are marked correct,
      b/c this is not the expected use of NOTA and really, it should
      still be written as a choice (with a clue) if this is the case.
(3) Choose randomize, NOTA and limit 3/4/5, check that it works
-- Correct answer is no longer guaranteed to be among the answers
-- Selecting NOTA and another option returns invalid / empty
-- Selecting NOTA when a correct answer is shown returns wrong
-- Selecting NOTA when no correct answers are shown returns correct
-- Selecting NOTA when correct shows all correct answers (should we?)
--- a little shaky, might show 3 correct answers, only one checked

Reviewers: justinjaffray

Reviewed By: justinjaffray

Differential Revision: http://phabricator.khanacademy.org/D7487