Expression entry box should always be left-to-right, even in RTL languages.

Summary: Fixes T2091

Test Plan:
- Go to /math/cc-sixth-grade-math/cc-6th-expressions-and-variables/cc-6th-writing-expressions/e/writing-expressions-with-variables-word-problems?lang=he-il
- Type in the answer box
- Text should flow left-to-right

Reviewers: alpert

Reviewed By: alpert

CC: alpert

Maniphest Tasks: T2091

Differential Revision: http://phabricator.khanacademy.org/D6174