Template graph scaffolding

[nikolas]

This is still in progress, but just laying out a few more things as I want to keep the ball rolling here. I have the expression inside the new template graph class - next step is just to render it in the jsxgraph view.

[Evan-CTL]

I'm going to hold off on merging what's here. Merging into production wouldn't add any tangible difference and there are some warnings being thrown that I'm worried might trigger an issue.

Keep in mind that the expression still isn't saved anywhere.

[nikolas]

Alright, so this is ready for another review.

Current state is pretty basic function display with the x variable, but it's a good starting point we can build off of:

Still TODO:

• Expression should only be re-parsed onblur rather than onchange... or whatever 3demos does.
• More user-friendly front-end validation. Again, using 3demos as reference here.
• Expression isn't saved to the back-end yet. I really just wanted to focus on the front-end functionality here as that's what's novel, but obviously we will be adding a new expression column to the Graph model, at least.

Random notes:
Just importing mathjs in econpractice was causing a pretty strange bug with the decimal.js dependency of math.js. See: josdejong/mathjs#3165

I am currently working around this by including my custom fork of decimal.js: nikolas/decimal.js@76c195d

mathjs proved to be difficult to make a custom build of, but this solution is satisfactory.