Adds support for text input readonly mode.

Test Plan:
Viewed text box in sat section review mode (which toggles readonly mode
too) and could not change the value.
Created numeric-input widget in Perseus editor and observed no
problems.

Reviewers: dylan

Reviewed By: dylan

Subscribers: kitt, aria

Differential Revision: http://phabricator.khanacademy.org/D14441