Use the mode instead of the sass variable to determine which color the
buttons in a toolbar should be based on the background color. Added
error messages when anything other than a color is passed and more
comments.
BREAKING CHANGES: ios mode `$toolbar-ios-button-color` now has a
default value of `color-contrast($colors-ios, $toolbar-ios-background,
ios)` which will evaluate to the primary color for light background
toolbars and white for dark background toolbars.
fixes#6364
BREAKING CHANGES:
`$toolbar-md-button-color` no longer gets passed to the function that
sets the contrast color for toolbar buttons, but it can still be used
to set the default button color.
`$bar-button-md-color` was renamed to `$toolbar-md-button-color`
`$bar-button-md-border-radius` was renamed to
`$toolbar-md-button-border-radius`
references #6364
this adds the functions necessary for the other modes as well
BREAKING CHANGE:
Can now pass contrast to the colors map:
```
$colors-ios: (
primary: (
base: #327eff,
contrast: yellow
),
secondary: (
base: #32db64,
contrast: hotpink
),
danger: #d91e18,
light: #f4f4f4,
dark: #222
) !default;
```
references #5445
