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/course.go

@@ -39,6 +39,11 @@ type Course struct {
 	SortOrder     int       `gorm:"column:sort_order"`
 	CreatedAt     time.Time `gorm:"column:created_at"`
 	UpdatedAt     time.Time `gorm:"column:updated_at"`
+
+	// Readings and Sessions are the hasMany relations the course form's
+	// relation managers edit.
+	Readings []Reading      `gorm:"foreignKey:CourseID"`
+	Sessions []ClassSession `gorm:"foreignKey:CourseID"`
 }
 
 // TableName is the plugin table.