Add LinkedIn and Youtube to footer

Summary: See title. Also cleans up some configs that are unlikely to be used anytime soon.

Test Plan:
make serve
Navigate to http://127.0.0.1:4000/ and see that the new footer links work and look good.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7616