Summary: This diff implements Gmails "load images, always load images from bengotow@gmail.com" option. Someone asked for it late last night and I figured it'd be fun to add. We also needed to refactor the MessageItem to allow for a GPG plugin - MessageItems now subscribe to the body of the message from the messageBodyProcessor, so in the event that processing rules change, someone can invalidate the processor cache by calling `resetCache()`, and then it recomputes bodies and triggers a refresh of each message body.
Test Plan: Updated existing tests, no new tests for this plugin just yet.
Reviewers: evan, juan
Reviewed By: evan
Differential Revision: https://phab.nylas.com/D2235
