Feature: MathQuill-basic

`make basic` now builds `mathquill-basic.{js,min.js,css}` and `font/Symbola-basic.{eot,ttf}`, a stripped-down version of MathQuill for basic mathematics, without advanced LaTeX commands. Specifically, it doesn't let you type LaTeX backslash commands with `\` or text blocks with `$`, and also won't render any LaTeX commands that can't by typed without `\` (omit `LatexCommandInput.js`, `advancedSymbols.js`, and `text.js`). The resulting JS is only somewhat smaller, but the font is like 100x smaller. (TODO: reduce full MathQuill's font size.)

Prep: split up `symbols.js` into `{basic,advanced}Symbols.js` (after some re-organizing), separate out `LatexCommandInput` from `commands.js`, and move `{Math,Text}Field` out of `publicapi.js` to `{math,text}.js` (respectively).

Also, `make test` builds `mathquill-basic.{js,css}` too, so that `unit.html` can include it and `publicapi.test.js` can unit-test it, and so that a new `basic.html` can visual-test it. (Needs whole new test page to check that `mathquill-basic.css` including the `Symbola-basic` font files works, which for simplicity uses the same @font-face font name `Symbola`.)

Closes #290, which is about configuring typing `\` to insert `\backslash`.
Closes #306, which is about separating out commands that can only be typed with `LatexCommandInput`; now separated as `advancedSymbols.js`.
Supersedes #318 (with additional fixes).