Responsive layout fix

[marcysutton]

To address issues in #962, this PR includes a style refactor for mdToolbar and markup changes in the docs.

An update to the toolbar icons will come later as part of #916 (removing icon text and updating artwork to be more semantic).

Closes #962

[gkalpak]

The implementation and semantics of hide-lt-md are wrong.
If nothing else it sounds more like a hide-lt-lg.

This again brings up the issue raised in #772 (which has been long ignored/"neglected"), namely:
show-/hide-{size} attributes are implemented in a way that is (more or less) wrong.

[marcysutton]

Yes, I had trouble figuring out what attributes to use. The combination I thought would accomplish what I wanted (hide something when the screen is less than 960w) was [hide][show-gt-md] but it didn't didn't hide anything, if I remember correctly. If there's an attribute combination that accomplishes what I'm after without adding a new attribute, sweet! If we can keep this issue moving forward without being delayed by another outstanding one, that would be ideal.

[mzbyszynski]

@marcysutton hide-sm hide-md should work, if you don't mind the verbosity...

[marcysutton]

Thanks–I've updated the code to work without adding a new attribute. They could definitely be more intuitive.