refactor: Clean up golden-layout css (deephaven#1322)

We maintain our golden-layout package, bake our theme customizations
directly into the dark theme. Eliminates some css. Used e2e testing to
prevent visual regressions, the two changes showing are intentional.

Had to exclude the ThemeExport from the karma tests, it didn't like the
scss import.

---------

Co-authored-by: Mike Bender <mikebender@deephaven.io>

packages/dashboard/src/PanelPlaceholder.scss

@@ -4,9 +4,13 @@
   display: flex;
   flex-direction: column;
   justify-content: center;
-  padding: $spacer-3;
   min-height: 100%;
   max-height: 100%;
   text-align: center;
   overflow: hidden;
+
+  // add padding on an inner wrapper so it is included in max height of parent
+  div {
+    padding: $spacer-3;
+  }
 }

packages/dashboard/src/PanelPlaceholder.tsx

@@ -11,7 +11,7 @@ const PanelPlaceholder = React.forwardRef(
     const component = LayoutUtils.getComponentNameFromPanel({ props });
     return (
       <div ref={ref} className="panel-placeholder">
-        Component &quot;{component}&quot; is not registered.
+        <div>Component &quot;{component}&quot; is not registered.</div>
       </div>
     );
   }