[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).

Updated unit tests to account for this.
[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, except test build

'make test' avoids using the define wrapper, so that the original test
suite will run properly.

Also
- Stub the noConflict function so it doesnt modify window properties
- Add the commit sha1 to the build header
[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 latex cmd for times symbol ×
[LRN] Add extra math symbols

\xrightarrow
\xleftarrow
[LRN] Add support for double prime symbol
[FEATURE] Support custom unitalicized text blocks

https://app.asana.com/0/7141602072726/15767912475323/f
[BUG] Missing refactor from Eoghan not committed version
[LRN] Fixed \x(right|left)arrow symbol with response rendering
[LRN] Making matrix RegExp non greedy, so it allows few matrices
[LRN] Fixing modifications because of removal initExtractContents:

https://github.com/mathquill/mathquill/commit/b90df1b33a0276e2e7a1e4394c31487339c1c5e8
[LRN] Selecting & copying text fixed for Learnosity additions:
- InertMath
- StaticMathWithEditables
[BUG] Mathquill: make approaches within limits display inline
[LRN] Fixing matrices bug, after changing _.edited to _.reflow
[LRN] Fixing bug in InertMath - it wasn't rendered at all
[LRN] Added to new symbols

- circled dot
- parallelogram