Add default status unpublished for newly created runs.

Add default status unpublished for newly created course runs, also update
existing course runs with null/empty status to unpublished.

LEARNER-5202

course_discovery/apps/course_metadata/migrations/0083_auto_20180511_1406.py

+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.11 on 2018-05-11 14:06
+from __future__ import unicode_literals
+
+import djchoices.choices
+from django.db import migrations, models
+
+from course_discovery.apps.course_metadata.choices import CourseRunStatus
+
+
+def change_runs_null_state_to_unpublished(apps, schema_editor):
+    CourseRun = apps.get_model('course_metadata', 'CourseRun')
+    for course_run in CourseRun.objects.filter(status=''):
+        course_run.status = CourseRunStatus.Unpublished
+        course_run.save()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('course_metadata', '0082_person_salutation'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='courserun',
+            name='status',
+            field=models.CharField(choices=[('published', 'Published'), ('unpublished', 'Unpublished')], db_index=True, default='unpublished', max_length=255, validators=[djchoices.choices.ChoicesValidator({'published': 'Published', 'unpublished': 'Unpublished'})]),
+        ),
+        migrations.RunPython(change_runs_null_state_to_unpublished, migrations.RunPython.noop),
+    ]

course_discovery/apps/course_metadata/models.py

@@ -451,8 +451,8 @@ class CourseRun(TimeStampedModel):
     uuid = models.UUIDField(default=uuid4, editable=False, verbose_name=_('UUID'))
     course = models.ForeignKey(Course, related_name='course_runs')
     key = models.CharField(max_length=255, unique=True)
-    status = models.CharField(max_length=255, null=False, blank=False, db_index=True, choices=CourseRunStatus.choices,
-                              validators=[CourseRunStatus.validator])
+    status = models.CharField(default=CourseRunStatus.Unpublished, max_length=255, null=False, blank=False,
+                              db_index=True, choices=CourseRunStatus.choices, validators=[CourseRunStatus.validator])
     title_override = models.CharField(
         max_length=255, default=None, null=True, blank=True,
         help_text=_(