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/less/_base.less

@@ -236,6 +236,7 @@ a,
 .ui.breadcrumb a {
   color: var(--color-primary);
   cursor: pointer;
+  text-decoration-skip-ink: all;
 }
 
 a.muted {
@@ -245,7 +246,19 @@ a.muted {
 a:hover,
 a.muted:hover,
 .ui.breadcrumb a:hover {
-  color: var(--color-primary-dark-2);
+  color: var(--color-primary);
+}
+
+a.label,
+.repository-menu a,
+.ui.search .results a,
+.ui .menu a,
+.issue-keyword a {
+  text-decoration: none !important;
+}
+
+.ui.breadcrumb a:hover {
+  text-decoration: underline !important;
 }
 
 .ui.breadcrumb .divider {

web_src/less/_repository.less

@@ -2837,20 +2837,6 @@ tbody.commit-list {
   }
 }
 
-.commit-summary a {
-  &:hover {
-    text-decoration: underline solid;
-  }
-
-  &.default-link {
-    text-decoration: none;
-
-    &:hover {
-      text-decoration: underline solid;
-    }
-  }
-}
-
 .commit-list .commit-status-link {
   display: inline-block;
   vertical-align: middle;
@@ -3012,13 +2998,12 @@ td.blob-excerpt {
   background-color: #fafafa;
 }
 
-.issue-keyword,
-.commit-body .issue-keyword:hover {
-  border-bottom: 1px dotted var(--color-text-light-2);
+.issue-keyword {
+  border-bottom: 1px dotted var(--color-text-light-3) !important;
 }
 
 .issue-keyword:hover {
-  border-bottom: none;
+  border-bottom: none !important;
 }
 
 .file-header {

web_src/less/helpers.less

@@ -6,6 +6,7 @@
 .ac { align-items: center !important; }
 .tc { text-align: center !important; }
 .tl { text-align: left !important; }
+.tdn { text-decoration: none !important; }
 .jc { justify-content: center !important; }
 .js { justify-content: flex-start !important; }
 .je { justify-content: flex-end !important; }