fix(list): fixed list icon spacing to work with sass

I'm not sure why, but `sass` renders this as:

```
--rmd-icon-text-spacing: calc(
  var(--rmd-list-text-keyline, 4.5rem)-var(--rmd-list-item-horizontal-padding, 1rem) -
  var(--rmd-icon-size, 1.5rem)
);
```

Since there is no space between the two `var` calls in the calc, it
treats the value as `0`. This update makes it so `sass` renders this
correctly as:

```
--rmd-icon-text-spacing: calc(
  var(--rmd-list-text-keyline, 4.5rem) -
  var(--rmd-list-item-horizontal-padding, 1rem) -
  var(--rmd-icon-size, 1.5rem)
);
```

Note: Added newlines for readability.

I also checked the other usage of `calc` + css variables throughout
react-md, and all of the others seem to work correctly. Super weird.

Closes #1015

packages/list/src/_mixins.scss

@@ -165,12 +165,12 @@
 ///
 /// @param {String|Number} subtract - The amount that should be subracted
 @mixin rmd-list-item-addon-spacing($subtract) {
-  $to-keyline: #{rmd-list-theme-var(text-keyline)} - #{rmd-list-theme-var(
-      item-horizontal-padding
-    )};
+  $keyline: rmd-list-theme-var(text-keyline);
+  $padding: rmd-list-theme-var(item-horizontal-padding);
+
   @include rmd-icon-theme-update-var(
     text-spacing,
-    calc(#{$to-keyline} - #{$subtract})
+    calc(#{$keyline} - #{$padding} - #{$subtract})
   );
 }