Management command to check imap config emails

[payamnj]

Courses can optionally add IMAP server configuration so their users can interact by sending emails instead of web interface. Actions that user can perform by sending an email are:

1- Registering for a course by sending an email with subject register <course_slug>
2- Unsubscribe from a course by sending an email with subject cancel <course_slug>
3- Verify their subscription by sending an email with subject verify <otp_code>
4- Get a list of courses table provided by organization by sending an email with subject list courses

The task for this ticket is to write a management command to check unopened emails from all the define IMAP connections, parse if the email subject has a valid command the execute the relevant command handler.