This patch removes Bootstrap classes from the footer in favor of
[flexbox](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout/Basic_Concepts_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.