Feature: support for plugins to add menu items

[cc-a]

Description

We're currently exploring adding functionality with additional view functions via a plugin. For these to be accessible to the end user, we'd like to make them available in the navbar as a separate menu item.

Component

No response

Additional information

Perhaps the simplest way to implement this would be to provide an empty `block` in `common/authorized_navbar.html` so that the existing template can be easily extended.