Bulk Email: Add design styling

Switch to using decorators; refactor and cleanup tests.

lms/djangoapps/bulk_email/models.py

@@ -10,6 +10,7 @@ class Email(models.Model):
     Abstract base class for common information for an email.
     """
     sender = models.ForeignKey(User, default=1, blank=True, null=True)
+    # The unique hash for this email. Used to quickly look up an email (see `tasks.py`)
     hash = models.CharField(max_length=128, db_index=True)
     subject = models.CharField(max_length=128, blank=True)
     html_message = models.TextField(null=True, blank=True)
@@ -24,10 +25,20 @@ class CourseEmail(Email, models.Model):
     """
     Stores information for an email to a course.
     """
-    TO_OPTIONS = (('myself', 'Myself'),
-                  ('staff', 'Staff and instructors'),
-                  ('all', 'All')
-                  )
+    # Three options for sending that we provide from the instructor dashboard:
+    # * Myself: This sends an email to the staff member that is composing the email.
+    #
+    # * Staff and instructors: This sends an email to anyone in the staff group and
+    #   anyone in the instructor group
+    #
+    # * All: This sends an email to anyone enrolled in the course, with any role
+    #   (student, staff, or instructor)
+    #
+    TO_OPTIONS = (
+        ('myself', 'Myself'),
+        ('staff', 'Staff and instructors'),
+        ('all', 'All')
+    )
     course_id = models.CharField(max_length=255, db_index=True)
     to = models.CharField(max_length=64, choices=TO_OPTIONS, default='myself')