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 {
+  padding: $gl-vert-padding $gl-padding;
+  line-height: 36px;
+
+  .controls {
+    margin-top: -2px;
+    float: right;
+  }
+
+  .dropdown-menu-toggle {
+    line-height: 20px;
+  }
 
-  .panel-heading {
-    padding: $gl-vert-padding $gl-padding;
-    line-height: 36px;
-
-    .controls {
-      margin-top: -2px;
-      float: right;
-    }
-
-    .dropdown-menu-toggle {
-      line-height: 20px;
-    }
-
-    .badge {
-      margin-top: -2px;
-      margin-left: 5px;
-    }
-
-    &.split {
-      display: flex;
-      align-items: center;
-    }
-
-    .left {
-      flex: 1 1 auto;
-    }
-
-    .right {
-      flex: 0 0 auto;
-      text-align: right;
-    }
+  .badge {
+    margin-top: -2px;
+    margin-left: 5px;
   }
 
-  .panel-empty-heading {
-    border-bottom: 0;
+  &.split {
+    display: flex;
+    align-items: center;
   }
 
-  .panel-body {
-    padding: $gl-padding;
+  .left {
+    flex: 1 1 auto;
+  }
 
-    .form-actions {
-      margin: -$gl-padding;
-      margin-top: $gl-padding;
-    }
+  .right {
+    flex: 0 0 auto;
+    text-align: right;
   }
+}
+
+.panel-empty-heading {
+  border-bottom: 0;
+}
+
+.panel-body {
+  padding: $gl-padding;
 
-  .panel-title {
-    font-size: inherit;
-    line-height: inherit;
+  .form-actions {
+    margin: -$gl-padding;
+    margin-top: $gl-padding;
   }
 }
+
+.panel-title {
+  font-size: inherit;
+  line-height: inherit;
+}