[FIX] Remove the unused approved field, this one has been replaced by the field 'state'.
[IMP] Update the signature of each email
[FIX] Force the use of SSL/TLS
[FIX] There was an error during the formating of the subject of the email
[IMP] Add a new command to send the invitation to the speakers.
[IMP] Define a new config key for the emails
[REF] Move the mail_message function from manage.py to pythonfosdem/tools.py file
[REF] In pythonfosdem/bp_general.py, Use the new function from pythonfosdem.tools