🗃️(core) add is_gradable boolean field to CourseRun model

This boolean field allow a course leader to mark its session as gradable.
Gradable course runs can be used to generate a certificate.

src/backend/joanie/core/migrations/0002_courserun_is_gradable.py

@@ -0,0 +1,18 @@
+# Generated by Django 4.0.3 on 2022-03-18 16:56
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("core", "0001_initial"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="courserun",
+            name="is_gradable",
+            field=models.BooleanField(default=False, verbose_name="is gradable"),
+        ),
+    ]

src/backend/joanie/core/models/courses.py

@@ -267,6 +267,7 @@ class CourseRun(parler_models.TranslatableModel):
         choices=lazy(lambda: ALL_LANGUAGES, tuple)(),
         help_text=_("The list of languages in which the course content is available."),
     )
+    is_gradable = models.BooleanField(_("is gradable"), default=False)
 
     class Meta:
         db_table = "joanie_course_run"