Add hints
Multiple question types
Hint for second question type
Minor rewrite of  hints
Create exercise
Simplify exercise
Term.regex() can be treated by RationalExpression.regex()
Update rational expression unit test
Remove unnecessary extras to regex
Remove unnecessary extras to regex
Add conditions to final hint
Remove debugging
Remove review_mode from interface.js

Test Plan: Crossed fingers

Reviewers: benkomalo

Reviewed By: benkomalo

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D3793
Refactor exercises so hints are simpler
Remove extra ,

Test plan:
Crossed fingers, or really make lint

Auditors: alpert
Rename katex.less.css to just katex.css

So that a webapp `make clean` doesn't delete it.

Auditors: emily
Merge pull request #66520 from petercollingridge/add-rat-exp-3

Improve consistency of rational expression module
Merge pull request #66424 from petercollingridge/add-rat-exp-6a

Adding and subtracting rational expressions 6 (new branch)
Merge pull request #66604 from petercollingridge/sim-tri

Solving similar triangles 1.5
Add missing period

Auditors: peter