Remove BigSymbol::placeCursor #cleanup

I forgot to remove placeCursor when cherrypicking the desmos
customization.
https://github.com/mathquill/mathquill/pull/256#discussion_r10264620
Fix editing \int and move back to symbols.js

\int needs an html element
https://github.com/mathquill/mathquill/pull/256#discussion_r10244679
Cleanup setting BigSymbol up/down pointers

https://github.com/mathquill/mathquill/pull/256#discussion_r10264697
Use unicode char as key in LatexCmds
Change BigSymbol to SummationNotation

It's not really a symbol anymore.
Fix SummationNotation tests to use .write

https://github.com/mathquill/mathquill/pull/256/files#r10264955