Update nautobot-app for current design standards

[gsnider2195]

After the Load Balancers app virtualserver model is fully implemented, go back through the PRs and update the cookiecutter to match the current design standards and best practices where necessary. Examples:

• Delete tests/test_api_views.py
• Update TestCase base classes:
  • test_api.py should be nautobot.core.testing.api.APIViewTestCases.APIViewTestCase
  • test_filters.py should be nautobot.apps.testing.FilterTestCases.FilterTestCase
  • test_forms.py should be nautobot.apps.testing.FormTestCases.BaseFormTestCase
  • test_models.py should be nautobot.apps.testing.ModelTestCases.BaseModelTestCase
• Add the attributes to the test classes that must be filled out for the generic tests to work (APIViewTestCase.choices_fields, APIViewTestCase.create_data, PrimaryObjectViewTestCase.form_data, etc)
• Add a sample setUpTestData to the API and view tests with a comment explaining that 3 instances must be created in order for the generic deletion tests to work
• Fix MRO of the example filterset in filters.py
• The example model form Meta.fields should be __all__
• The example filterset Meta.fields should be __all__
• We should probably add the minimum recommended extras_features for the example model definition
• The url patterns should be using the model's plural name. We should probably add a comment explaining the correct pattern (virtual-servers instead of virtualserver)