[FIX] web: BS4, fix custom radio button style

This is a standard BS4 bug: radio buttons's dot is not properly centered
as soon as the html font-size is not a multiple of 4 (and our backend
uses 13px). As a quick fix, I change that to 12px and use a font-size base
of 13/12rem so that the body font-size does not change. Bootstrap will
probably fix this in a future update.

addons/web/static/src/scss/primary_variables.scss

@@ -4,8 +4,8 @@
 ///
 
 // Font sizes
-$o-root-font-size: 13px;
-$o-font-size-base: 1rem;
+$o-root-font-size: 12px;
+$o-font-size-base: 13rem * (1px / $o-root-font-size);
 $o-line-height-base: 1.5; // This is BS default
 
 // Colors