Adopt SummerCMS v0.1.1 admin features

Datepickers on native time.Time / lagoon.Date fields, file uploads on
materials, readings and class sessions managed from the course form
(course_id made nullable).

models/courses/fields.yaml

@@ -56,3 +56,9 @@ fields:
         label: jz.bm::lang.common.description
         type: textarea
         size: large
+    readings:
+        type: relation-manager
+        relation: readings
+    sessions:
+        type: relation-manager
+        relation: sessions