Show a message with a selectable background below the header but above the breadcrumb, e.g. to announce a coming maintenance or ongoing incidents.
Optionally these messages can be made expiring on a given date and can be dismissed on a per-user basis.
The message is formatted using the global markup formatter (see Manage Jenkins > Security > Markup Formatter). Depending on the configured formatter it may contain html, e.g. to add a link.