RHOAIENG-20553: fix(jupyter/utils/addon) fix vertical centering of the PatternFly spinner

After we switched to the css without global css reset, we no longer have the rule

```
  :where(html, body) {
    height: 100%;
  }
```

present, and it causes the spinner to be tucked to the top edge of screen.

I've checked that setting this does not visibly change how JupyterLab (with TensorBoard open) displays.

Author: jiridanek

jupyter/utils/addons/partial-head.html

@@ -3,7 +3,11 @@
   type="text/css"
   href="{{page_config.fullStaticUrl}}/pf.css"
 />
+<!-- set height to 100% so that vertical centering works -->
 <style>
+  :where(html, body) {
+    height: 100%;
+  }
   #loading-container {
     width: 100%;
     height: 100%;