Avoid rewriting admin.register() to decorator after unregister

[adamchainz]

Description

Currently the new admin.register fixer from #182 moves register() calls after unregister() calls:

+@admin.register(User)
 class CustomUserAdmin(...):
     ...

 admin.site.unregister(User)
-admin.site.register(User, CustomUserAdmin)

This is unsafe!

Update the fixer to detect unregister calls and avoid rewriting.

@andreasschmitz