Keep Trix dynamic styles in the head

Trix dynamically adds some styles to the head that we need to keep around.
Otherwise, the editor will not render correctly after a page change.

We can detect those styles because Trix also adds a `data-tag-name`
attribute to the style elements it adds.

Ideally, we would move those styles to Trix's CSS file, but for now we'll
just skip removing them. We don't want to force everyone upgrading to
Turbo v8 to also upgrade Trix.

Ref:

- https://github.com/basecamp/trix/blob/06d8b1db5fb682d007c5ca041884f6297674c8b7/src/trix/elements/trix_editor_element.js#L108-L162
- https://github.com/basecamp/trix/blob/06d8b1db5fb682d007c5ca041884f6297674c8b7/src/trix/core/helpers/custom_elements.js#L11

src/core/drive/page_renderer.js

@@ -114,7 +114,12 @@ export class PageRenderer extends Renderer {
 
   removeUnusedHeadStylesheetElements() {
     for (const element of this.unusedHeadStylesheetElements) {
-      document.head.removeChild(element)
+      // Trix dynamically adds styles to the head that we want to keep around
+      // Long term we should moves those styles to Trix's CSS file
+      // but for now we'll just skip removing them
+      if (!element.dataset.tagName) {
+        document.head.removeChild(element)
+      }
     }
   }