Change <a> elements to underline on hover (#17898)

Fomantic brings a opinionated style that removed underline on mouse
hover which I think is important UX to have.

This re-enables the underline in the Fomantic config and fixes a few
cases where underline was deemed disruptive.

web_src/fomantic/_site/globals/site.variables

@@ -6,3 +6,4 @@
 @useCustomScrollbars: false;
 @disabledOpacity: var(--opacity-disabled);
 @variationPopupTooltip: false;
+@linkHoverUnderline: underline;