Make "Select all that apply" bold and italic

Summary:
Chris and Lori have been pointing out that many students don't notice
the "select all that apply prompt", and that it would help to be bolded.
To make this distinct from the actual problem prompt text, I made it
italic as well (thought open to other thoughts here).

Test Plan:
 * Make a radio widget
 * Check 'Multiple Selections'
 * Verify that "Select all that apply" is in bold and italic.

Reviewers: alex, eater

Reviewed By: eater

Subscribers: khirasaki, jack

Differential Revision: http://phabricator.khanacademy.org/D8902