Tweak breakpoint mixins

[flachware]

Minor issue: I didn't have a closer look by now, but I noticed that the from() mixin in _respond.scss adds 1px, which leads to a min-width of 769px for tablets – so the iPad would not be included.

I guess we could just not add that 1px?

[tnajdek]

Certainly, happy to revisit. Do you reckon having overlapping from and upto makes sense?

[flachware]

No, we should avoid overlapping.

E.g.: 'tablet': 768

upto => tablet – 1px from => tablet between => small, tablet – 1px

(Similar to the Bootstrap 3 logic)