First step to refactor the .hide to .gt-hidden (#22916)

A separate PR from #22884 （without touching the jQuery methods)

web_src/less/_base.less

@@ -1807,16 +1807,9 @@ footer {
   }
 }
 
+// TODO: refactor to use ".gt-hidden" instead (a simple search&replace should do the trick)
 .hide {
   display: none;
-
-  &.show-outdated {
-    display: none !important;
-  }
-
-  &.hide-outdated {
-    display: none !important;
-  }
 }
 
 .center:not(.popup) {

web_src/less/_review.less

@@ -57,7 +57,6 @@
 .show-outdated,
 .hide-outdated {
   &:extend(.unselectable);
-  display: block !important;
 
   &:hover {
     text-decoration: underline;

web_src/less/helpers.less

@@ -22,7 +22,6 @@
 /* below class names match Tailwind CSS */
 .gt-pointer-events-none { pointer-events: none !important; }
 .gt-relative { position: relative !important; }
-.gt-hidden { display: none !important; }
 
 .gt-mono {
   font-family: var(--fonts-monospace) !important;
@@ -181,3 +180,6 @@
   .gt-w-100-small { width: 100% !important; }
   .gt-js-small { justify-content: flex-start !important; }
 }
+
+// gt-hidden must be placed after all other "display: xxx !important" classes to win the hidden chance
+.gt-hidden { display: none !important; }