Switch default testing/development domain name to examplectf.com (#1807)

* Use `examplectf.com` as testing/development domain instead of `ctfd.io`

tests/api/v1/statistics/test_scores.py

@@ -22,7 +22,7 @@ def test_api_statistics_score_distribution():
 
         # Add user data
         register_user(app)
-        user = Users.query.filter_by(email="user@ctfd.io").first()
+        user = Users.query.filter_by(email="user@examplectf.com").first()
         simulate_user_activity(app.db, user=user)
 
         # Test again