Update Bootstrap to latest 3.x

[lucasrowe]

Before doing a bunch of redesign work, let's update to the latest Bootstrap 3.x.

This task has been rewritten to the latest 3.x instead of 4. I started working on 4 and decided that it'll be too much to take on right now, retrofitting pages.

[lucasrowe]

I'm going to try to cleanup some of the custom bootstrap stuff too while we're at it. The bootstrap project structure isn't super clear.