Indicator for admins about registrations pending approval

I've run into the following problem several times:
During the first few weeks of a semester, loads of people sign up for our JupyterHub instance and I can check the authorization page regularly to approve them easily. But as soon as this initial flood dies down, I often miss people who come in later and they have to wait very long to be approved because I don't check the authorization page often any more.

### Proposed change:
It would be great to have some sort of indicator displayed to admin users if there are pending requests for authorization, maybe even how many there currently are. This information should not be visible to the normal user.

I think it should be possible to work within the existing templating mechanism (cf. #79 for necessary changes for using templates this way) to achieve this, similar to how [announcements](https://jupyterhub.readthedocs.io/en/stable/reference/templates.html#page-announcements) work.