Maintain default data and implement app hooks

This MR probably have been split int two MRs. While it started out fixing the issue with default data vanishing on demo-master, it grew to ensure all apps get an API for declaring default data on any model, and hardening to ensure apps don't do silly mistakes that lead to inobvious problems.

It now implements default hooks for apps to use in a simple manner.

aleksis/core/migrations/0001_initial.py

@@ -14,6 +14,7 @@ class Migration(migrations.Migration):
 
     dependencies = [
         migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+        ('otp_yubikey', '0001_initial'),
     ]
 
     operations = [