Assigning multiple departments to a page as an administrator

### As an administrator, I want to easily be able to assign more than one department to a page, so that we maintain our polyhierarchical IA.

**Is your feature request related to a problem? Please describe.**
This is currently technically possible through groups (https://joplin-pr-3813-department-grou.herokuapp.com/admin/groups/)

If an admin wants to add a page to another department, they can go to the new department's group permissions page
(https://joplin-pr-3813-department-grou.herokuapp.com/admin/groups/3/) and then "Add a page permission" for the page they want to add to the department

However, it's a confusing experience. Let's make it better.

**Ideas**
- @briaguya suggests some of underlying group things on the department edit page, and avoid surfacing this altogether.
- maybe bring back editing related departments on a page level, but maybe not!

**Additional context**
Add any other context or screenshots about the feature request here.