Add category and item randomization options to categorizer widget

Summary: The categorizer editor now gives you the option to randomize the order of the items and the categories. When randomizing the categories you can also choose to keep the last category at the end of the list (e.g. if the last option is "None of these"). Fixes T2928.

Test Plan:
- Add a categorizer widget with multiple items and categories
- Toggle each of the randomization checkboxes in the editor, and observe randomization!
- For each permutation check that the correct answer is still graded as correct.
NOTE: If there are only a few options you're more likely to observe no-change in the randomization, but it appears as though it's really just shuffling to the same result.

Reviewers: jack, eater

Reviewed By: jack, eater

Subscribers: alex, erin, jack, eater

Maniphest Tasks: T2928

Differential Revision: http://phabricator.khanacademy.org/D11514