Backport panels styles from EE

See
https://gitlab.com/gitlab-org/gitlab-ee/blob/43a570dcf5cbd0a98572d5601e9000e906791db0/app/assets/stylesheets/framework/panels.scss

app/assets/stylesheets/framework/panels.scss

 .panel {
   margin-bottom: $gl-padding;
+}
+
+.panel-slim {
+  @extend .panel;
+  margin-bottom: $gl-vert-padding;
+}
 
-  .panel-heading {
+
+.panel-heading {
   padding: $gl-vert-padding $gl-padding;
   line-height: 36px;
 
@@ -32,23 +39,22 @@
     flex: 0 0 auto;
     text-align: right;
   }
-  }
+}
 
-  .panel-empty-heading {
+.panel-empty-heading {
   border-bottom: 0;
-  }
+}
 
-  .panel-body {
+.panel-body {
   padding: $gl-padding;
 
   .form-actions {
     margin: -$gl-padding;
     margin-top: $gl-padding;
   }
-  }
+}
 
-  .panel-title {
+.panel-title {
   font-size: inherit;
   line-height: inherit;
-  }
 }