* added threading for invitation emails to avoid wsgi timeouts for big elections
* added email success/failure feedback 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

* removed double shadow in the election view
* reduced default value for election to 5 min
* removed old todos

removed stusta domain references in the code and moved it to the config file, moved default email address (used when no manger email is set) to the settings file

issue #22: prints qr code of the meeting link onto the pdf when available, removed json export since its redundant and one small model label update

small fixes in session settings, added datetime picker in session settings, small ui fix in sessions view

moved template variable description from view to form in add election (like we did for add session) and added table-responsive for template description