[landing] Re-introduce hover states for social icons

Summary: Basically brought back what we had from before (with some minor tweaks): https://github.com/CommE2E/comm/blob/3a79d65da519971ea4f3000699f67fea75a9754c/landing/header.css

Test Plan:
Here's how it looks:

How it looks on iPhone 13 mini (layout):

Reviewers: benschac, def-au1t, palys-swm, varun, ashoat

Reviewed By: ashoat

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3408