refactor: move get_user_model call out of method

The get_service_user method used to do a local call to the
get_user_model function because it was not guaranteed to be properly
initialized at the time of import. This was partly due to how we did
custom initialization using lms/startup.py, but it was also because
when it was implemented (commit f318661), the platform was still
running on Django 1.8.18. At that time, get_user_model was guaranteed to
work only after Django has imported all models.

In Django 1.11, the behavior of get_user_model was changed:

  https://docs.djangoproject.com/en/1.11/releases/1.11/#django-contrib-auth

> get_user_model() can now be called at import time,
> even in modules that define models.

Now that lms/startup.py is gone and get_user_model is safe to call at
the module level, I'm refactoring the catalog app's models.py file to
follow the convention we use everywhere else in edx-platform with
respect to get_user_model.

openedx/core/djangoapps/catalog/models.py

@@ -1,6 +1,4 @@
 """Models governing integration with the catalog service."""
-
-
 from config_models.models import ConfigurationModel
 from django.conf import settings
 from django.contrib.auth import get_user_model
@@ -9,6 +7,8 @@
 
 from openedx.core.djangoapps.site_configuration import helpers
 
+User = get_user_model()
+
 
 class CatalogIntegration(ConfigurationModel):
     """
@@ -72,7 +72,4 @@ def get_internal_api_url(self):
         return helpers.get_value('COURSE_CATALOG_API_URL', settings.COURSE_CATALOG_API_URL)
 
     def get_service_user(self):
-        # NOTE: We load the user model here to avoid issues at startup time that result from the hacks
-        # in lms/startup.py.
-        User = get_user_model()  # pylint: disable=invalid-name
         return User.objects.get(username=self.service_username)