Issue #17: added threading for invitation emails and feedback for whether email sending failed

• closes #17 (closed)
• added threading for invitation emails to avoid wsgi/asgi timeouts for big elections
• added email success/failure feedback via for the sending of invitations in the session's management. This is done asynchronous with django channels and modals and alerts that show the failed email addresses
• failed email addresses are highlighted in red in the management frontend
• fixed pylint version
• generalized applicant as option for all situation where we dont know for sure whether its a person or not