Change `fluidContainer` variants to apply to the components + use existing variants keys for the utilities

[benface]

Now that Tailwind 1.5 has added a variants argument to addComponents(), it makes more sense for the fluidContainer variants key to be applied to the components. I’m realizing now that It never really made sense to apply these variants to the utilities; instead, we should look at the width, minWidth, maxWidth, padding, and margin variants keys.