Expose createGraphie function

Auditors: eater
Refactor label positioning metrics a little bit

Now it won't hang when a graphie is removed from the DOM before it
finishes render. Also should be a smidgen faster in general.

Auditors: eater
Use MathJax Process instead of Typeset

Typeset just calls PreProcess then Process and we don't need the steps
that PreProcess does (turning \( ... \) into script tags, etc.). This is
a little bit faster!

Auditors: eater
Add 'move' event on each movable point

Auditors: eater
Fix sliding-window exercises

Test Plan:
Loaded and dragged on recognizing_concavity.

Auditors: eater
Fix issue reporting

Auditors: emily