Upgrade Font Awesome to 5.x

Summary:
See title. This gives us more up-to-date icons in the footer
as well as access to some new coding-related icons that Font Awesome 4
does not have.

Depends on D7616

Test Plan:
make serve
Navigate to http://127.0.0.1:4000/ and verify the footer icons all
render well and with the new icons (notably Youtube).

Navigate to http://127.0.0.1:4000/doc/0.22.2/rpc/control/help/ and
http://127.0.0.1:4000/doc/release-notes/release-notes-0.22.2.html and
verify the nav icons render correctly.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7617