Closes #15042: Move model registration logic to AppConfigs

[jeremystretch]

Fixes: #15042

• Move code from extras.utils.register_features() into netbox.models.features._register_features()
• Rename netbox.models.features._register_features() to register_model()
• Decouple model registration from the class_prepared signal
• Call register_model() for each model under AppConfig.ready() for each app
• Extend PluginConfig.ready() to also call register_model() for each model
• Add a plugin test for model registration