ui: Add background-color: currentColor to all our icon masks (#7729)

We currently define our 'decorational' iconography in our CSS via
background images and or psuedo content. For coloring these we use
`mask-image` and a background color.

This commit adds a background-color: currentColor to our %with-mask
placeholder that makes the color of these icons default to the `color`
of the current element, meaning the icons now inherit from things like
`:hover`.

This can easily be overwritten as before by just setting the
background-color on the icon manually as before.

ui-v2/app/styles/base/icons/base-placeholders.scss

@@ -5,6 +5,7 @@
 %with-mask {
   mask-repeat: no-repeat;
   mask-position: center;
+  background-color: currentColor;
 }
 %as-pseudo {
   display: inline-block;