🚩(frontend) add feature flag on "Copy as HTML"

As a blue print, we add a feature flag on
"Copy as HTML" button in the doc toolbox.
This feature flag is controlled by the `CopyAsHtml`
feature flag.

Be aware:
- if the feature flag is disabled, the button
will be shown
- if the feature flag is enabled and send true,
the button will be shown
- if the feature flag is enabled and send false,
the button will be hidden

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to
 
 ## Added
 
+- 🚩(frontend) feature flag analytic on copy as html #649
 - ✨(frontend) Custom block divider with export #698
 
 ## Changed