Added an unread thread count to the web title Added useEffect hook to app.react.js to update title when count changes

Summary: Added an unread thread count to the web title. Added useEffect hook to app.react.js in order to keep title updated.

Test Plan: Tested in web and with flow. Created an additional account in order to see if the title count would update to (1). I also used the 'mark as unread' feature to make two of the threads unread, which would display (2). Made sure clicking an unread threat would reduce the unread title count. And I also made sure refreshing the page would show the correct unread count once it is loaded.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D620