Changed student query_key to a UUID. Closes #87

• Added stages to the subscription mechanism, meaning that a subscription is distingushed between feedback-creation, -validation and -conflict resolution
• Minor refactoring to the Feedback ViewSet and serializer
• Added a larger integration test

core/migrations/0001_initial.py

-# Generated by Django 2.0.1 on 2018-01-07 14:37
-
-import uuid
+# Generated by Django 2.0.1 on 2018-01-10 10:46
 
+import core.models
+from django.conf import settings
 import django.contrib.auth.models
 import django.contrib.auth.validators
+from django.db import migrations, models
 import django.db.models.deletion
 import django.utils.timezone
-from django.conf import settings
-from django.db import migrations, models
-
-import core.models
+import uuid
 
 
 class Migration(migrations.Migration):
@@ -113,13 +111,14 @@ class Migration(migrations.Migration):
                 ('subscription_id', models.UUIDField(default=uuid.uuid4, editable=False, primary_key=True, serialize=False)),
                 ('query_key', models.CharField(blank=True, max_length=75)),
                 ('query_type', models.CharField(choices=[('random', 'Query for any submission'), ('student', 'Query for submissions of student'), ('exam', 'Query for submissions of exam type'), ('submission_type', 'Query for submissions of submissions_type')], default='random', max_length=75)),
-                ('owner', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='susbscriptions', to=settings.AUTH_USER_MODEL)),
+                ('feedback_stage', models.CharField(choices=[('feedback-creation', 'No feedback was ever assigned'), ('feedback-validation', 'Feedback exists but is not validated'), ('feedback-conflict-resolution', 'Previous correctors disagree')], default='feedback-creation', max_length=40)),
+                ('owner', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='subscriptions', to=settings.AUTH_USER_MODEL)),
             ],
         ),
         migrations.CreateModel(
             name='StudentInfo',
             fields=[
-                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('student_id', models.UUIDField(default=uuid.uuid4, editable=False, primary_key=True, serialize=False)),
                 ('has_logged_in', models.BooleanField(default=False)),
                 ('matrikel_no', models.CharField(default=core.models.random_matrikel_no, max_length=8, unique=True)),
                 ('exam', models.ForeignKey(null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='students', to='core.ExamType')),