Migrate footer to flexbox

Summary:
This patch removes Bootstrap classes from the footer in favor of flexbox.

Flexbox gives us more fine-tuned control of the layout (compared with Bootstrap's grid system) and usually
results in a simpler DOM. This will allow us to fix varying instances of overlapping nav elements and
mis-sized containers throughout the site.

This is a good place to start for the flexbox migration since the footer is pretty well self-contained.

Test Plan:
make serve
Navigate around and make sure nothing is broken.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7400