Fixes the variables for input font size

Created by: andresgalante

This PR changes the variable for the form control to use a scoped component variable instead of a global font size variable.

scss/_forms.scss

@@ -9,7 +9,7 @@
   width: 100%;
   height: $input-height;
   padding: $input-padding-y $input-padding-x;
-  font-size: $font-size-base;
+  font-size: $input-font-size;
   font-weight: $input-font-weight;
   line-height: $input-line-height;
   color: $input-color;