Fix up `max-width` with `-min` variable usage - smaller than `$screen-xs-max` breakpoint

The following discussion from gitlab-ce!11841 should be addressed:

  • @annabeldunstone started a discussion: (+5 comments)

    Mixing max with a min variable usually isn't a good idea, but it might be okay in this case since there isn't a width smaller than xs-min, so it'll never be off-by-one. @iamphill what do you think?

Full discussion:

Screen_Shot_2017-06-01_at_1.28.11_PM

Edited by Annabel Dunstone Gray