Summary:
- Simplify undoManager to just maintain the undo/redo history items
- DraftEditingSession manages snapshotting state of draft, hack allows it to also save selection (still hoping to eventually put selection in body HTML as markers)
- Switch from `debounce` to `throttle` style behavior so typing for along time followed by undo doesn't undo away your entire block.
This resolves two issues:
+ Changes to participant fields are no longer undoable because they go straight to the session.
+ Changes to metadata weren't undoable.
Test Plan: Tests WIP
Reviewers: evan, juan
Reviewed By: juan
Differential Revision: https://phab.nylas.com/D2956
