Fix up warnings

- the default auto-increment primary key field type is now
  configurable, and Django's check command issues a warning if you are
  just assuming the historical behavior of using AutoField.

- Django 3.2 brings in automatic AppConfig discovery, so all of our
  explicit `default_app_config = ...` assignments in __init__.py
  modules are no longer needed, and raise a RemovedInDjango41Warning.

awx/settings/defaults.py

@@ -41,6 +41,11 @@ else:
 DEBUG = True
 SQL_DEBUG = DEBUG
 
+# FIXME: it would be nice to cycle back around and allow this to be
+# BigAutoField going forward, but we'd have to be explicit about our
+# existing models.
+DEFAULT_AUTO_FIELD = 'django.db.models.AutoField'
+
 DATABASES = {
     'default': {
         'ENGINE': 'django.db.backends.sqlite3',