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/bootstrap.scss

@@ -5,6 +5,7 @@
  */
 
 // Core variables and mixins
+@import "functions";
 @import "variables";
 @import "mixins";