As an initial step to surface cross-wiki notifications, we can show an indicator of notifications existing in another project, that leads to that project. This won't result int he most fluent experience, but will help with the user navigation and awareness of external notifications, and other tasks will be created for upcoming iterations (more details in T114350).
**Acceptance criteria:**
# If the user does not have unread notifications in other wikis, the usual notification panel is shown without anything different.
# If there are unread notifications, a special notification item is shown to make the user aware of it.
## The indicator will tell the user about the notifications from one project ("10 notifications from Commons")
## If the user clicks on the notification, the user will navigate to the main page of such project with the notification panel open. Which will allow to read those notifications.
## If the user discards (or reads) the notification, another one will be shown for the project with the most recent notifications (if any).