This PR adds a small visual change to the new feature item on the `/admin/whats-new` page. When features are marked with an experimental site setting, they should show an indication on the feature item that it is "Experimental"
Bug introduced in this PR https://github.com/discourse/discourse/pull/29244
When the experiment toggle button was introduced, new features did not look right when the toggle button was not available.
In addition, the plugin name can be an empty string. In that case, information about new features should be displayed.
Toggle the button to enable the experimental site setting from "What's new" announcement.
The toggle button is displayed when:
- site setting exists and is boolean;
- potentially required plugin is enabled.
