Move the user & group administration views to the NetBox UI

### NetBox version

v3.5.1

### Feature type

New functionality

### Proposed functionality

Introduce designated UI views for user and group administration, and remove these from the Django admin UI.

### Use case

The admin UI is being removed in NetBox v4.0 (see #12325).

### Database changes

_No response_

### External dependencies

_No response_