Implement themed enrollment email feature

Course instructors can choose to send email to users who enroll before or after the course start date. This option is in the Settings -> Schedule & Details page in studio, and is available by setting the ENABLE_ENROLLMENT_EMAIL feature flag.

Allows theming the default enrollment email body by specifying
DEFAULT_PRE_ENROLLMENT_EMAIL and DEFAULT_POST_ENROLLMENT_EMAIL
in environment settings file.

Moves bulk_email models to common in order for studio to access the CourseEmailTemplate model, for sending test email in studio.

*Note @griffresch @jinpa per discussion, this is behind a flag since eventually it would be a global platform feature rather than course-specific.
These docstrings should describe the behavior that would lead to `None` being returned.