* Clean up some utilities
- Align CSS properties
- In `_spacing.scss`, we had a comment saying what 'a' was for, but we removed that so this comment no longer applies
* Remove '-xs' from `.navbar-toggleable-xs` and remove duplication
* Fix outdated classes to use newer ones
* Explore responsive display utils, but with a twist: lowest breakpoint has no breakpoint modifier in the class name
* make floats use the same format, add float-none mixin
