[web] Set document title in App component

Summary: In https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=1272%3A88750 we're introducing a new navigation menu which will be rendered conditionally instead of AppSwitcher. Setting document title should always happen and that requires moving this functionality back (D2881) to the App.

Test Plan: Mark a thread as unread and check if the number in title was increased. Mark as unread and check if it decreased.

Reviewers: benschac, atul, def-au1t, ashoat

Reviewed By: benschac, ashoat

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3337