v4: The Utilities Update

Addressing a ton of things in one massive PR around our utilities. We've already broken each group of utilities out into new pages with #20068, and now I'm cleaning things up to make utilities more granular, easier to use, and more flexible to a multitude of use cases.

• Changed syntax of spacing utilities to help avoid collision with Microformats (see #20185 (closed)). This doesn't completely resolve the problem, however, because I want to move from .pa-*/.ma-* to just .p-* and .m-* to avoid a weird and unclear abbreviation of "all" there.
• Revamps the text and background color utilities. Background utilities no longer set a color; you now apply that with a separate utility for more control. Also new is a .text-white for those common cases where you just want white text on dark background.
• Renames .img-rounded an .img-circle to .rounded and .rounded-circle. Also, add new utilities for rounding a complete side of an element (e.g., .rounded-top or .rounded-right). (Pulls in commits from #20363 to fix #20266 (closed).)
• Removes display: block; from .img-fluid. Responsive image behavior is not dependent on display: block;, so we can safely remove it on our end. Should you need block level, you can easily override that in the source or with a utility class.

Still to do:

• Rename the .pull-* classes to reflect their CSS property, .float-*.
• Update all instances of background utilities to ensure color contrast. Punting.
• Consider reorganizing these new utility docs a bit more (e.g., combine display and responsive display, combine floats and clearfix, or something to that effect). Punting.
• Update all URLs that link to and within the new utility docs.
• Audit and make consistent use of !important.
• Add vertical alignment utilities Punting to 4.1 ideas in #20866 (closed)

Am I missing anything else major?