Refactored and renamed grid mixins

I don't know if this pull request is welcome, but I've refactored and renamed some grid mixins.

I think they are more descriptive  (e.g. `.loop-grid-columns` is much clearer as `.make-grid` in my opinion). And I also DRY'ed up the generation of screen size dependend grids (xs, sm, md, lg) with a new mixin.