v4: Grids

[mdo]

Meta tracking again.

• v4 - No flexbox grid documentation #17119: Missing flexbox docs
• v4 - Need example of vertical alignment. May be need component. #17122: Vertical alignment
• Add auto-width columns for flexbox grid #17131: Flex auto width?
• Change .col-{size}-{n} to .col.{size}-{n} #17228, v4: Tweak predefined grid classes #17261: New base class?
• Grid system broken with flex and IE10-11 #17310: IE10 and IE11 are broken with flex
• Customizable grid gutters per breakpoint #17481: Custom grid gutters per breakpoint
• 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: Mixin changes
• .container, .container-fluid should be conditional upon $enable-grid-classes #17586: Skip containers when grid classes are disabled
• Document that @screen-*-min were replaced by $grid-breakpoints #17593: Document variables changes in migration
• Why I need to write col-xs-12 if I want full column width on mobile with Flexbox grid? #17603: use of -12 with flex?
• v4 firefox responsive images not responsive in flexbox layouts. #17694: Fluid images and flex don't mix in FF
• Flexbox grid column wrapping bug #17756: Flex wrapping bug
• Including "bootstrap-grid.scss" only leads to somewhat broken grid #17982: Broken grid when only using grid file?
• flex - classes with justify-content #18024: Flex and justify
• Remove useless .col-*-offset-12 and .col-xs-offset-0 classes #18036: Drop -offset-12?
• sm container width is larger than minimum sm viewport width #18054: Container widths greater than media queries
• v4 - Add more flex grid helper classes #18164: More flex helpers
• Add completely flush containers (with no padding) #18207: Flush containers?
• Create "block-grid" like Foundation? #18407: Block grid?
• Incorrect line wrapping in grid row with .col-xs-1, .col-xs-11 in Firefox with Flexbox enabled #18420: Firefox 1/11 flex column setup wraps

[cvrebert]

• add min-width to container #18746: Minimum container/viewport width?

[mdo]

Not everything is done here, but closing out for no as the bulk is being handled in #19099.