[keyserver] Redirect to the right store depending on the user agent

Summary: Use detect-browser to determine what OS user is using and redirect to App Store or Google Play Store.

Test Plan: Check on physical iOS and Android emulator if clicking /invite/:secret link opens the right store.

Reviewers: bartek, kamil, ashoat

Reviewed By: ashoat

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7240