Aligned some editor events, Changed wysiwyg custom styles loading

- Removed old 'editor-*-update' commands to instead use the aligned
  'editor::replace' command that we already have.
- Changed the way custom styles are loaded for the WYSIWYG editor so we
  don't need an API call but instead scape content from the parent page
  header using comments as identifiers. Added tests to ensure comments
  exist and align.

resources/js/components/markdown-editor.js

@@ -98,11 +98,6 @@ class MarkdownEditor {
             toolbarLabel.closest('.markdown-editor-wrap').classList.add('active');
         });
 
-        window.$events.listen('editor-markdown-update', value => {
-            this.cm.setValue(value);
-            this.updateAndRender();
-        });
-
         this.codeMirrorSetup();
         this.listenForBookStackEditorEvents();