Slow typing performance on long emails

[sbrudz]

While testing the last few pull requests, I've noticed that typing into long emails is very laggy. I profiled the code and found that the checkForWarnings function in just-not-sorry is the culprit. It adds and removes the warnings each time you type a character. This causes Chrome to recalculate the layout of the DOM over and over again, which is slow when there are a lot of nodes being impacted. We should implement a less expensive method of handling warnings to address this issue. The changes needed to fix this issue could also serve as a base for being able to implement #59 .

[maniacialexa]

In progress - 2 days thus far writing tests because we were missing test coverage. About another day of work.