Based on T115690: Add a customizable field for text or template on top of Flow boards.

It is sometimes important to have global notices on the top of a Flow board:

• for an article, any warnings concerning article content "this page is not the [subject]'s official fan/costumer service/contact page"
• for a user, a way to pass a message, like "I'm on a wikibreak"
• for a project, the possibility to display a "read-first".