Add a min-height: 1.5rem; to custom controls

Fixes #20730.

This change computes the minimum needed height of what a single line of text would be for the custom checkboxes/radios. This is required because our custom control indicators are positioned absolutely, meaning they cannot be clearfixed or anything like that. Using a computed value means it should scale nicely in case of customization

scss/_custom-forms.scss

@@ -12,6 +12,7 @@
 .custom-control {
   position: relative;
   display: inline-block;
+  min-height: (1rem * $line-height-base);
   padding-left: $custom-control-gutter;
   cursor: pointer;