Avoid automatically importing Branch & StagedChange models in `nbshell`

[jeremystretch]

NetBox version

v4.1.1

Feature type

Change to existing functionality

Proposed functionality

When invoking a Python shell via manage.py nbshell, do not automatically import the Branch and StagedChange models from extras.

Use case

These models are seldom used, will soon be deprecated (see #17472), and conflict with the Branch model provided by the netbox-branching plugin (which aims to replace the core Branch model).

Database changes

N/A

External dependencies

N/A