[Button] Disabled button mixin does not respect "opacity" argument

[emprice]

Expected Behavior

When the button-disabled mixin is included in Sass, the button should get the opacity given in the $opacity argument, using $button-opacity-disabled as a fallback.

Current Behavior

Currently, button-disabled sets the opacity to $button-opacity-disabled no matter what value is passed for $opacity.

Possible Solution

The mixin simply doesn't read its argument and is set up to use the default no matter what. The fix is trivial:

// scss/components/_button.scss, line 283
@mixin button-disabled(
  $opacity: $button-opacity-disabled
) {
  // previously: opacity: $button-opacity-disabled;
  opacity: $opacity;
  cursor: not-allowed;
}

Steps to Reproduce

How to reproduce:

1. Add @include button-disabled to a Sass rule
2. Compile stylesheet with Sass and link in a document

Context

Nothing fancy, just using the more fine-grained mixins to customize the appearance of my web app. In its current form, the mixin won't work for me, because I don't set the value of $button-opacity-disabled before importing Foundation.

Your Environment

• Foundation version(s) used: 6.7.5, via npm
• Browser(s) name and version(s): Firefox
• Device, Operating System and version: Ubuntu

Checklist

• I have read and follow the CONTRIBUTING.md document.
• This is a bug report or a feature request.
• There are no other issues similar to this one.
• The issue title and template are correctly filled.