Remove fomantic dimmer module (#30723)

Tested extensively using modal which is the only dependant.

web_src/css/base.css

@@ -680,10 +680,6 @@ input:-webkit-autofill:active,
   box-shadow: 0 6px 18px var(--color-shadow) !important;
 }
 
-.ui.dimmer {
-  background: var(--color-overlay-backdrop);
-}
-
 .ui.dropdown .menu > .header {
   font-size: 0.8em;
 }

web_src/css/index.css

@@ -16,6 +16,7 @@
 @import "./modules/table.css";
 @import "./modules/card.css";
 @import "./modules/checkbox.css";
+@import "./modules/dimmer.css";
 @import "./modules/modal.css";
 
 @import "./modules/select.css";

web_src/css/modules/dimmer.css

@@ -0,0 +1,30 @@
+/* These are the remnants of the fomantic dimmer module */
+
+.ui.dimmer {
+  position: fixed;
+  display: none;
+  top: 0;
+  left: 0;
+  right: 0;
+  bottom: 0;
+  background: var(--color-overlay-backdrop);
+  opacity: 0;
+  z-index: 1000;
+  overflow-y: auto;
+  justify-content: center;
+  padding: 8px 0;
+  animation-name: fadein;
+  animation-duration: .2s;
+  user-select: none;
+}
+
+.ui.active.dimmer {
+  display: flex;
+  opacity: 1;
+}
+
+.ui.dimmer > * {
+  position: static;
+  margin-top: auto !important;
+  margin-bottom: auto !important;
+}