Account for disabled buttons

* https://github.com/Versal/versal-gadget-api/issues/29
* https://versal.atlassian.net/browse/GAD-2://versal.atlassian.net/browse/GAD-21

I was following examples set in the .com repo:
https://github.com/Versal/versal.com/blob/master/client/style/src/styles/button/button.styl#L75