Add material design icons to admin/settings views (#27780)

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

app/javascript/styles/mastodon/admin.scss

@@ -10,6 +10,13 @@ $content-width: 840px;
   width: 100%;
   min-height: 100vh;
 
+  .icon {
+    width: 16px;
+    height: 16px;
+    vertical-align: middle;
+    margin: 0 2px;
+  }
+
   .sidebar-wrapper {
     min-height: 100vh;
     overflow: hidden;