[web] Remove userAgent from Redux state

Summary: Same as the previous diff, basically. We should've had this in a React context... basically wanted server code to know what the user agent was while doing SSR. Now we don't do SSR, and we can just call detectBrowser() without needing to pass anything in.

Test Plan: Flow

Reviewers: ginsu, michal, atul, tomek

Reviewed By: ginsu, michal, tomek

Differential Revision: https://phab.comm.dev/D5985