Enable fluid page layout on medium size viewports (#21178)

Fomantic has abrupt breakpoints at 991px and 768px which leads to
variable amounts of wasted screen space below those breakpoints.
Instead, enable fluid width for all viewport sizes below 1200px.

web_src/less/_base.less

@@ -737,6 +737,13 @@ a.ui.card:hover,
   padding-bottom: 80px;
 }
 
+/* enable fluid page widths for medium size viewports */
+@media @mediaMdAndUp and @mediaLgAndDown {
+  .ui.ui.ui.container:not(.fluid) {
+    width: calc(100vw - 3em);
+  }
+}
+
 .following.bar {
   z-index: 900;
   left: 0;