Fix the centered column floating glitch by targeting specific selector, instead of using !important.

Should fix both #4200 and #4624.
Moving CSS rules that are irrelevant to $size out of the mixin grid-html-classes($size).
Adding size-specific centered & uncentered styles into @grid-html-classes mixin.
Avoid introducing new variables
Merge pull request #5186 from MrOrz/master

Remove `!important` and reorder certain styles in grid system to fix centered grid