Fix broken links in link tracking and read receipts
Fix bug in email frame where it wouldn't adjust the height even when
content changed
MessageItem bodies automatically clear the MessageBodyProcessor cache when
the message contents (including metadata) change.
Remove unused Account stuff from nylas-observables
Plugins appIds properly read if there's an environment set
Summary:
Adds two (very similar) plugins - Open Tracking and Link Tracking.
Both can be enabled via a button in the composer. Open tracking
inserts a tracking pixel right before send, and link tracking replaces
all links with tracked redirects. Both plugins use the new Metadata
service to store their open/click counts, and have backend servers to
respectively serve the pixel image or handle the redirects. Requests
also trigger a metadata update to increment the open/click counters.
Test Plan: Manual for now
Reviewers: evan, bengotow, drew
Reviewed By: bengotow
Differential Revision: https://phab.nylas.com/D2583
