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

Closes #17

See merge request !36

channel redis is needed for redis backend deployment

* 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

Feature reload

Closes #15

See merge request !35

Sort election in a more advanced way

See merge request stustanet/wahlfang!34

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

spectator links for elections, issue #25

See merge request stustanet/wahlfang!32

Explain green mail addresses, issue #24

See merge request stustanet/wahlfang!31

export prometheus metrics

Closes #12

See merge request stustanet/wahlfang!30

resolves issue #6: generalization of applicants to options except self apply is activated

Closes #6

See merge request stustanet/wahlfang!29

multiple doc improvements

See merge request stustanet/wahlfang!28

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