* 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
* make spacer utils responsive by grid tier
* update scale to add two levels, document them
* change responsive spacing utils to avoid the xs abbreviation in the class name for that tier
* update code snippet to match source
* update usage in our docs
* linter
* docs updates
- Move and rename .img-rounded to .rounded, .img-circle to .rounded-circle
- Add new .rounded-{direction} utils
- New docs pages for border utils with TBD comments for the border property
- Removes most image examples for rounding from the content/images docs in favor of new docs page
- Rather than mix multiple properties in our color utilities, this splits all color and all background utils into separate classes.
- Adds new .text-white class to help lighten text color for darker backgrounds
