[ENH] - Filter users and groups a user can share an app with based on JupyterHub permissions

[aktech]

Feature description

After #224 is merged, we'll have the ability to share apps with a set of users and groups.

We need to filter the users and groups in the UI based on the permissions user has, as in a user may only have permission to share with only a certain set of users and groups.

Value and/or benefit

When sharing an app the users and groups that are displayed in the UI are basically all users and groups, If a user tries to share the app with someone they don't have permission to share with, the sharing will fail silently.

Anything else?

No response