[LRN] Add StaticMathWithEditables (for editable areas)

Replicates the behaviour of the old mathquill where the outer
area isn't editable once you add editable zones.

I needed to add an extra class to distinguish inner editable areas,
and the last focused area (so we know where to write when adding
math programmatically).

Arrow keys move the cursor between the editable zones.

https://github.com/Learnosity/API/commit/68d28f93daf85f09035865965f06b2b58abce53d
https://github.com/Learnosity/API/commit/1e098271bcf14331160c01588212a502bef6fa47
[LRN] Check textarea can be selected before trying to do so

Otherwise it'll break when using substituteTextarea to use
a focusable span.
[LRN] Disable tab and shift-tab behaviour

Because we want to be able to tab between the mathquill editor
area and an input widget (formula block keyboard)
[LRN] Allow direct entry of $ (dollar sign)

By removing the binding that creates a text block
when $ is pressed.
[LRN] Add extra math symbols

\abs
\nparallel
\measuredangle
\overrightarrow
\overleftarrow
[LRN] Change \triangle and \ddots character bindings

\triangle is now a bigger triangle
\ddots now points down and right
[LRN] Automatically add sub block for limits
[LRN] Wrap in define call & own scope
[LRN] Fix bug selecting from lower lines

E.g. the bottom part of a fraction, vector, matrix

https://github.com/mathquill/mathquill/issues/312
[LRN] Add char cmd for times symbol ×