Adding helper classes for breakpoint display

[sbsmith86]

Added some helper classes we can use to display things at specific breakpoints. I added most of the ones that I think would be more common.

@DFurnes @weerd

[DFurnes]

Yaaaas! :boat: :boat: :boat:

[weerd]

Maybe as a future upgrade these could be classes that refer to a mixin... cause say we want to .show-at-medium, but instead of display: block we need to have it display: table or display: inline-block. It could be a mixin that defaults to display: block, but we can pass another type of display value if needed? Thoughts?

[sbsmith86]

@weerd interesting, is it better that way instead of just overriding the class as we need?

[DFurnes]

Another option is just to adjust the media queries to only target the places we want to display: none, rather than writing the classes mobile-first.

[weerd]

Let's pop this through for now to get the ball rolling, but we should come back and ponder some more. :dancers:

[sbsmith86]

:+1: