Switch up some split button stuff

- New vars
- New function for stripping units so we can combine rems and ems in math functions
- Add new classes for sizing and spacing around the split dropdown toggle so that the caret isn't misaligned

scss/_functions.scss

@@ -0,0 +1,17 @@
+//
+// Functions
+//
+
+
+// Remove the unit of a length
+// Source: https://css-tricks.com/snippets/sass/strip-unit-function/
+//
+// @param {Number} $number - Number to remove unit from
+// @return {Number} - Unitless number
+
+@function strip-unit($number) {
+  @if type-of($number) == 'number' and not unitless($number) {
+    @return $number / ($number * 0 + 1);
+  }
+  @return $number;
+}