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).

controllers/materials.go

@@ -29,11 +29,8 @@ func (materialsAdmin) RequiredPermissions() []string {
 	return []string{PermMaterials}
 }
 
-// AdminFieldRelations binds the course picker and the tags.
-//
-// TODO: requires SummerCMS change. Materials carry their files as
-// system_files attachments (field "files"), but cabana has no file upload
-// form widget, so files are attached outside the admin form for now.
+// AdminFieldRelations binds the course picker and the tags. The files field
+// is bound by models.Material.AttachRelations.
 func (materialsAdmin) AdminFieldRelations() []cabana.FieldRelationContract {
 	return []cabana.FieldRelationContract{
 		courseRelation("course_id"),