Adding scrollbar to header, only on mobile

[maazadeeb]

Motivation

Fixes #441. This has a different implementation than #444 in order to avoid #445. Only in the case of when screen size is lesser than 735px, we need a scrollbar.

Test Plan

1. Add many dummy entries into the siteConfig.js.
2. Use the Chrome devtools' "Toggle Device Mode" to simulate various responsive sizes. Note: Just make sure you refresh the page once when you switch to mobile mode. This guarantees that the page is loaded in the simulated mobile mode and any old CSS effects from the desktop mode are not into play.
3. Also, test the search in every mode so that Search input bug (site) #445 doesn't come back.

cc @JoelMarcey

[JoelMarcey]

Love it (again)! 👍