Resolve "get() returned more than one PersonPreferenceModel [Ticket#75959]"

Closes #633

See merge request !937

Merge remote-tracking branch 'origin/master' into 633-get-returned-more-than-one-personpreferencemodel-ticket-75959

More default schedules

See merge request !939

Update DOT to 1.7 and add background task

See merge request !938

# Conflicts:
#   CHANGELOG.rst

Resolve "After changing the user of a person, groups are not synced"

Closes #628

See merge request !935

Resolve "[PWA] Icon background/app background are red"

Closes #630

See merge request !934

Resolve "Use maskable icons"

Closes #543

See merge request !936

Resolve ""Edit group type" form shows "CSV import reference" field"

Closes #631

See merge request !933

Resolve "PDF generation broken with S3 storage"

Closes #629

See merge request !931

Update dependency django-cleanup to v6

See merge request !932

Works around an issue with boto3:

  https://github.com/jschneier/django-storages/issues/708

Prepare release 2.7

See merge request !929

Translations update from Weblate

See merge request !909

Currently translated at 100.0% (645 of 645 strings)

Translation: AlekSIS/AlekSIS-Core
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis-core/de/

Update translation files

See merge request !928