Update `nbshell` management command

[jeremystretch]

Proposed Changes

The nbshell management command is a wrapper for Django's stock shell. It's been a while since this has seen any substantial change, and is probably overdue for some updates. Some immediate items:

• Move APPS to constants.py
• Remove obsolete entries from EXCLUDE_MODELS
• List installed plugins on init
• Extend lsmodels() to include plugin models

Justification

Opportunistic cleanup