Rewrite exercise to mix number types and simplify
Add hints
Multiple question types
Hint for second question type
Fix denominator regex for add/sub rational expr 3

This is an ugly way to do it but it should hold for now.

Auditors: peter
Make volume_1 update the drawing in hints
Introduce an event for related videos being rendered.

Summary:
Since this is done asynchronously, it's useful for client code to know when
the UI is ready. This is immediately needed for a video efficacy experiment
that renders a guider, suggesting a user watch a related video.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D3779
Remove trailing whitespace.
Add a label for the `undefined` issues

Auditors: eater
Add explicit error for unknown problem type in dev

Otherwise you get a more-cryptic error later...

Auditors: emily
Fix drag behavior of 3D volume models in Firefox
Merge pull request #66227 from theinternetftw/volume-drag-fix

Fix drag behavior of 3D volume models in Firefox
Additional Firefox 3D model drag behavior fix
Merge pull request #66246 from theinternetftw/volume-drag-fix-2

Additional Firefox 3D model drag behavior fix
Fix data-fallback="0"

Silly jQuery...

Auditors: emily
extraneous_solutions: Use set the simpler way

set already doesn't care about order, so just use it as it was intended.

Test Plan:
Answered extraneous_solutions.html?seed=29&problem=ignore-extraneous with 7.

Auditors: peter
Remove spurious paren.

Auditors: alpert

Test Plan:
make i18n (in the main codebase)
Update update title tag script

Now it doesn't totally die on missing exercise files.

Auditors: eater
Update title tags
Change scratchpad bounds to be wider

Also tweaked the spacing a bit so it looks nicer.

Before:
{F4418}

After:
{F4419}

Auditors: alex