Insert comma to fix validation warning

BS doesn't support IE6/IE7, so comma can be inserted.
Combine separate efforts for grid mixins into one, cleanup and reorganize
restore var to customizer
Merge branch 'master' into restore_grid_mixins

Conflicts:
	dist/css/bootstrap.min.css
Merge branch 'master' into restore_grid_mixins

Conflicts:
	dist/css/bootstrap.min.css
Merge pull request #11050 from twbs/restore_grid_mixins

Restore grid mixins
add docs example for #10938 to show responsive column clearing in grid
follow up to #10102, cc @cvrebert
docs notice for #11010
mention rows must be in containers in grid docs
clarify more grid stuff, specifically 100% fluid designs; fixes #10711
Merge pull request #10955 from ZDroid/patch-1

Insert comma to fix validation warning
fix #10257
Merge branch 'master' of github.com:twbs/bootstrap
run grunt on that scaffolding change
move .hidden to utilities; fixes #10997