v4: Grid redux

[mdo]

Addresses a handful of grid changes:

• v4 - No flexbox grid documentation #17119: Stubs out docs for flexbox grid.
• Add auto-width columns for flexbox grid #17131: Adds auto layout for flexbox grid columns (to mixins and predefined classes) with the addition of .col-{breakpoint classes. For example, use .col-xs to create as many grid columns as you'd like from xs and up.
• v4: Tweak predefined grid classes #17261, Why I need to write col-xs-12 if I want full column width on mobile with Flexbox grid? #17603: Adds a new .col base class for grid columns and Changes the responsive grid modifier naming scheme from .col-{breakpoint}-{modifier}-{size} to .{modifier}-{breakpoint}-{size} for simpler grid classes. For example, instead of .col-md-3.col-md-push-9 it's col-md-3.push-md-9. Also updates all examples to new markup.
• Mixin make-col() adds float: left when this is not included in make-grid-columns() #17512, Add float in make-col-span() mixin, not make-col() #17514: Overhauls the grid mixins to merge make-col and make-col-span into a single make-col mixin, thereby ensuring mixins and predefined classes utilize the same float/flex behaviors.
• Document that @screen-*-min were replaced by $grid-breakpoints #17593: Adds basic grid migration documentation, including mention of flexbox support, flexbox utils, and variables/maps changes.
• flex - classes with justify-content #18024 , v4 - Add more flex grid helper classes #18164: Horizontal flexbox alignment utilities added.
• Document classes for vertical alignment of flexbox items #18691: Documents flexbox vertical alignment utilities.
• v4: Flex utilities instead of flex grid classes? #18785: Cherry-picks the changes to rename our flexbox grid utilities to global flexbox utilities.

Todo:

• Document flexbox utilities

[timalbert]

I don't think v4 documentation for modifier is updated