Resolve "Celery tasks are not working properly"

Closes #37

See merge request AlekSIS/official/AlekSIS-App-Untis!108

Do not include non-package files in wheel

See merge request AlekSIS/official/AlekSIS-App-Untis!107

Fix readme reference

See merge request AlekSIS/official/AlekSIS-App-Untis!106

Resolve "Allow configuring database options"

Closes #36

See merge request AlekSIS/official/AlekSIS-App-Untis!104

Resolve "Disambiguate long group names"

Closes #35

See merge request !103

Resolve "Delete deleted events"

Closes #34

See merge request !102

Resolve "Replace every occurrence of UNTIS by Untis"

Closes #30

See merge request !101

Resolve "move_dates_for_testing throws misleading errors"

Closes #33

See merge request !100

Add trademark hint to readme

See merge request !99

Update dev dependencies and reformat

See merge request AlekSIS/official/AlekSIS-App-Untis!98

Resolve "Classes without teachers break import"

Closes #32

See merge request !97

Add makemessages env to tox.ini

See merge request !96

Bump development version to 2.1.dev0

See merge request !95

Update changelog from 2.0

See merge request !94

(cherry picked from commit 69336497)

Resolve "Management commands are not run in the frontend"

Closes #29

See merge request !91

Update poetry.lock

See merge request !92