Restrict user's ability to fire off thousands of duplicate requests by holding down the Enter key (or just smashing their mouse)

Summary:
See http://phabricator.khanacademy.org/D3654 — we have some problemlog entities w/ thousands and thousands of attempts. Although this would still be possible w/ this change,
it'll be significantly harder and more time consuming for users to blast us with tons of /attempt API calls w/out modifying their attempt's content.

Test Plan:
Log in
Open any exercise on the learning dashboard
Enter incorrect answer
Hold down Enter key
Verify that only one request is sent when Enter key is first held down
Click Enter repeatedly and quickly
Verify that only one /attempt request is sent on first click
Verify that rapidly attempting different answers sends /attempt requests for every attempt
Verify that submitting an answer once, quickly hitting "Hint," and quickly submitting the same answer again submits /attempt requests for all three of those actions

Reviewers: alpert

Reviewed By: alpert

CC: benkomalo

Differential Revision: http://phabricator.khanacademy.org/D3658