Determine active editable area based on activeElement

Previously we were caching the block id of the last used
editable area. This caused a race condition when multiple
mathquill editor instances appeared on the same page.
Use bigger triangle symbol
Allow insertion of spaces

Useful when user attempts to enter content like
x\frac{1}{2}
Pressing a space after the x in this example
allows the fraction to be inserted separately.

Spaces in outputted latex now need to be cleaned up.
Add custom 'response' symbol to be added