scope customizer h4 change
Remove unneeded comment in layouts
Massive customizer audit and overhaul

I probably missed a few things and screwed up a few others, but this
takes the place of much of #9910. I haven't added many variables—that
should be reserved for v3.1—but I have rejiggered the layout, added
missing ones, corrected wrong values, etc.

/cc @cvrebert @juthilo @saas786
Merge branch 'master' of github.com:twbs/bootstrap