MathQuill-basic links to smaller Symbola-basic font

Font is linked by stylesheet, so now can't share stylesheet with full
MathQuill, `make basic` now has to build a totally separate stylesheet
mathquill-basic.css, by having font.less choose what font file to
reference based on a compile-time parameter.

Since `make basic` now builds minified JS + CSS + fonts, it's really
more like `make all` than like `make js`, so move it next to `all`.

Less doesn't actually have compile-time parameters, but you can set
variable values with --modify-var [1] (equivalent to putting appending
a variable declaration to the base file, and variables are lazy-loaded
so the last declaration is the one used [2]), then use a conditional
mixin to choose what font file to reference [3]. --modify-var was
released in v1.5.1 [4], so set that to the required version of Less
(was originally edge to fix some bug [5] but that was pre-1.3, should
be fixed by now).

[1]: http://lesscss.org/usage/#command-line-usage-modify-variable
[2]: http://lesscss.org/features/#variables-feature-lazy-loading
[3]: http://lesscss.org/features/#mixin-guards-feature-conditional-mixins
[4]: https://github.com/less/less.js/commit/daec7dff1c5d533bfaaee5315aa20c9bf56f1873
[5]: https://github.com/mathquill/mathquill/commit/411a37641fbcddd5c73268863dc49a53124189a5#commitcomment-6589168

Only add and reference .eot and .ttf files for now, even though for full
Symbola we currently have .eot, .ttf, .otf, .woff, *and* .svg files.
Desmos only has .eot, .ttf, and .woff files (presumably because .ttf and
.otf have the same browser support [6] and only old Opera and old iOS
Safari need SVG instead [7]), but all user agents that support .woff [8]
also support .ttf (which isn't true the other way around!), so, should
be fine. In fact, I probably actually want to trim some of the redundant
full Symbola font formats, but I'm putting that off for now, we'll fix
that when we fix our outdated @font-face syntax [9].

[6]: http://caniuse.com/tff
[7]: http://caniuse.com/svg-fonts
[8]: http://caniuse.com/woff
[9]: Currently using "the local reference" http://www.paulirish.com/2009/bulletproof-font-face-implementation-syntax/

In both full MathQuill and MathQuill-basic the @font-face font name (as
opposed to font filename) is just "Symbola", wouldn't want to have to
change the @symbola etc variables too, but that means there has to be a
separate test page for MathQuill-basic where the full Symbola font isn't
loaded, to check that the Symbola-basic font files load correctly.