integrate scheduler in docker example deployment

See merge request !1598

rm scheduler settings from development

See merge request !1612

keep scheduler appointments from beeing re-created immediately

See merge request !1610

add recruitment cleanup view

See merge request !1609

1830 appointment number in list

See merge request !1608

Some views crash if scheduler is not available.

It should not be necessary to run a scheduler during development

When the ContactView is submitted with a status other than `INVITED`,
the following steps are executed:

-   The `pre_save` signal is triggered, deleting the appointments that
    belong to a schedule.
-   The Participation itself is saved.
-   The appointments as defined in the form are saved.

The last step will restore the appointments that have been deleted in
the first one. This commit fixes that.

adapt setup.py

See merge request !1604

adapt demo users to changed group names

See merge request !1606

delete scheduler token on participation delete

See merge request !1605

ignore .git in flake8

See merge request !1607

contact view: stay on page on submit

See merge request !1601

Scheduler integration

See merge request !1593

1819 translations

See merge request !1599

update dependencies

See merge request !1600

https://docs.djangoproject.com/en/dev/releases/3.1.4/

Session.schedule_id field

See merge request !1596

fail silently on sending mails

Closes #107

See merge request !1597

convert settings-related template tags to context processor

See merge request !1595

replace os.path() with Path everywhere

See merge request !1594