Areas are multiples of 1/2, not always integers

Closes #38953.

Auditors: michaelchang
Disable autocapitalization on iOS devices. (Fixes: #39000)

Summary: Also, use <input type="number"...> on the radical answer-type.

Test Plan: Monitor bugs reported on khan-exercises.

Reviewers: eater, alpert

Reviewed By: alpert

CC: tom, stephanie

Differential Revision: http://phabricator.khanacademy.org/D1070
Fix radical input on touch devices.

Auditors: alpert, eater
Brit's dice probability exercise originated by David Sissitka, improved by Kitt, Eater, and Alpert

Test Plan: Local

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D1075
Merge branch 'master' of git://github.com/Khan/khan-exercises