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
