Updated drawio tinymce plugin to use embeds

- Adds support for handling drawings as embeds, based on image
  extension.
- Adds additional attribute to drawio elements within editor to prevent
  tinymce replacing embeds with a placeholder.
- Updates how contenteditable is applied to drawio blocks within editor,
  to use proper filters instead of using the SetContent event.

resources/sass/_tinymce.scss

@@ -48,6 +48,11 @@ body.page-content.mce-content-body  {
   display: none;
 }
 
+// Prevent interaction with embed contents
+.page-content.mce-content-body embed {
+  pointer-events: none;
+}
+
 // Details/summary editor usability
 .page-content.mce-content-body details summary {
   pointer-events: none;