Summary:
ignores composition event commands until they're done. We then simply
update the new state after that happens.
Some additional refactoring:
- The <Contenteditable /> prop is 'value' instead of 'html' to make it
look more like a standard React controlled input
- Removed `filters` prop and `footerElements` prop from Contenteditable.
These could easily be moved into the composer (where they belong).
- Moved contenteditable and a few of its helper classes into their own
folder.
- Moved `UndoManager` up out of the `flux` folder into `src`. Currently
undo/redo is only in the composer when all contenteditables should have
the basic funcionality. Will refactor this later.
- Fix tests
Test Plan: manual
Reviewers: bengotow
Reviewed By: bengotow
Differential Revision: https://phab.nylas.com/D2211
