Feature summary (what you would like to be able to do and where): Integrate a way for creators of a sandbox preview of a Central Notice banner (i.e. in a user page or mediawiki page) so that Central Notice requesters can view the preview without waiting for a Central Notice admin to configure it. This could be as simple as a clear off-meta environment for testing (i.e. on [https://www.mediawiki.org/wiki/Beta_Cluster Beta Cluster]).
Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
*As an organizer for a campaign, I have a web designer providing me graphics and other design artefacts, however, they are not a trusted user that should have Central Notice rights. They give me a design for the banner, and I have to share it with the Central Notice admin, who puts it in the banner software. It performs unexpectedly, and the designer needs to access the underlying code to diagnose the underlying issue. (see an example here).
*As an organizer, I want to understand what is possible in the banners, so I try editing a sandbox one. In the process I realize that my expectations are too complex for the suggested banner, and I talk with the CN Admin about using a simple template .
*As an affiliate or international campaign coordinator, I have multiple designed spaces on and off wiki that use the same code and design choices. I want to be able to build a "Central Notice template" for my future central notice activities as part of a package of assets for my programs or for "sub campaigns" (see convo here).
Benefits (why should this be implemented?):
*Central Notice administrators are volunteers and not assumed to be web-design experts, and shouldn't be expected to have the diagnostic skills to handle the visual design diagnostics.
*Designers of banners may not need, want or be trusted with the ability to schedule banners -- reserving those additional functions for Central Notice Admins allows more volunteers with different skills to address the problem.
This is related to https://phabricator.wikimedia.org/T230857