Summary:
Threads can now be opened in separate windows. This can be done via the popout
icon next to the print icon, or by double-clicking the thread when in double-
pane mode. Note that the single-click action is still fired, which is why
double-clicking does not work in single-pane mode. The popout icon changes to a
pop-in icon while in the popout window, to allow users to collapse it back into
the main window.
Test Plan: Tested locally
Reviewers: evan, juan
Reviewed By: juan
Subscribers: sdw
Differential Revision: https://phab.nylas.com/D3332