Bootstrap: v4.0.0-beta Release

It's a beta! Holy crap this took forever. Thanks for sticking with me on this one—means a lot, y'all <3.

Highlights!

• Dropped Normalize.css as a dependency, forking some of it and remixing it with our own Reboot for a more stable normalization approach.
• Update navbar responsive behaviors to be mobile first, use a new class name .navbar-expand instead of .navbar-toggleable, remove toggler position modifiers (relies on flex utils now), and fix a handful of bugs.
• Stabilize some grid behaviors and update the docs.
• Updated the grid to drop push, pull, and offset in favor of new .order- modifiers and margin utilities.
• Renamed tons of variables (see #22414 and #22092) for a more consistent naming scheme.
• Removed remaining cursor: pointers to restore browser defaults and better indicate actions vs hyperlinks.
• Renamed flexbox order utilities to better match utility naming scheme of .property-value.
• Restored Alpha 5's placement of _custom.scss in our import order.
• Rewrote display and responsive utilities to de-dupe a bunch of functionality.
• Removed Gruntfile and docs from package manager builds.
• Overhauled our color system with new Sass maps, variables, classes, and component variants.
• Updated browser support to bring ranges up to more recent versions.
• Added a bash script for linting unused variables.
• Moved jQuery and Popper to peerDependencies as they're not requirements for every part of Bootstrap.

Be sure to read the full ship list.