Browser Extension Install Page Design Polish

Created by: Joelkw

We built the install page for the browser extension and have some additional tweaks to make it friendlier.

Exploration: https://www.figma.com/file/V5t7KOzC6PIX75L4dz8IFc/RFC-221-Browser-extension-improve-discoverability-and-awareness-Approved?node-id=914%3A2364

(TODO @rrhyne to attach a figma link and/or context when ready.)