Use "hint", not "step" terminology

Summary: Beth explained that this is more appropriate for the humanities, and it doesn't seem any worse for math.

Reviewers: eater, elizabeth

Reviewed By: elizabeth

CC: elizabeth

Differential Revision: http://phabricator.khanacademy.org/D3276
Better escaping for TeX resulting from user-entered input.

Test Plan:
Tested on localhost, works the same.

Auditors: alpert
Avoid expressions and simplify too much
Sort out all the merges of rational-expression.js
Update unit tests
factor function renamed getTermsGCD
Make texCleanup always return this

Summary: Since cleanupMath checks if MathJax is `null`, we don't need to
check manually, and now we'll always return `this`, which was causing
some issues.

Auditors: alpert
Make non-latex labels work

Auditors: alpert
Fix category mode in radio

Auditors: alpert
Use mathjax to render big fractions

Summary: The numerator of the fractions was large enough that it was
wrapping around, and it looked bad (see T1243). Use MathJax instead :(

Auditors: alpert
Merge pull request #57865 from petercollingridge/multi-rat-1

Avoid expressions that simplify too much
Show messages for incomplete answers.

Test Plan: Previewed a Perseus exercise to make sure it worked.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D3288
Add failing radio category unit test

It passes on my machine with super secret local changes though...

Auditors: emily
Fix ensure in linear_equations_4

The data-ensure wasn't working because NUMERCOEFF wasn't defined at the time that the ensure was being run. Moving it a bit earlier fixes things.

Fixes #58048.

Auditors: peter
Rewrite rectchart to take an optional stroke

Like

```
rectchart([2, 3], [BLUE, null], 0, [null, GRAY]);
```

Auditors: eater
Fix shifting_and_reflecting_functions

I have no idea how this was working before, it seems to me like it would
have forced regeneration before and frozen things...

Auditors: alpert
Fix up radio, add some more unit tests

Summary: Make radio actually act the way it's supposed to

Test Plan: Run the tests, see that they work.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D3300
Tweak styles so that <form> contains the problem

Auditors: jakesandlund
Ensure the correct answer is always a choice

Auditors: emily