Move view function into own file, add test

tests/app/main/views/test_api_keys.py

@@ -0,0 +1,14 @@
+from tests.app.main import create_test_user
+from flask import url_for
+
+
+def test_should_show_api_keys_and_documentation_page(notifications_admin,
+                                                     notifications_admin_db,
+                                                     notify_db_session):
+    with notifications_admin.test_request_context():
+        with notifications_admin.test_client() as client:
+            user = create_test_user('active')
+            client.login(user)
+            response = client.get(url_for('.api_keys', service_id=123))
+
+        assert response.status_code == 200