Mark global variable assignments

These are all definitely assignments to global variables (that's the point of this mixin), so I've changed them. But when I started looking through the deprecation warnings, most of them seemed like the right thing. That is, most of the warnings are about variables called either "$box" or "$columns" that *should* be locals based on what the code seems to be doing. So marking those global would probably be a mistake. I think part of the issue is that in many cases it is not clear whether the variable in question is a global, or a local with the same name as a global. 

A good example is the "nested" mixin, in _context.scss. Should the variable "$columns" in this mixin be used as a global, or a local? It's unclear, because this mixin calls @content, and I am unsure whether a variable scoped to the nested mixin would be visible to the @content block, or it would need to be global.

Please let me know if I can be of more help than this. I don't feel like I achieved much here.