Closes: #282 - Add handling for max_custom_object_types config option (and configuration.md)

[bctiemann]

Closes: #282

Enables configuration of a max_custom_object_types setting:

PLUGINS_CONFIG = {
    'netbox_custom_objects': {
        'max_custom_object_types': 1000,
    }
}

Attempting to create more than this number of Custom Object Types will raise a ValidationError (this is done via clean at the model level, so this will work the same via the UI or API).

[jeremystretch]

I would add max_custom_object_types to default_settings in the AppConfig as well, as a supplementary means of documentation.

We might also want to set a default limit (e.g. 100) for sanity.

[jeremystretch]

We decided to lower the default maximum to 50.

[jeremystretch]

We decided to lower the default maximum to 50.