allow to clear datetime widget

See merge request !1876

use subqueries for complex excludes

See merge request !1881

see note in https://docs.djangoproject.com/en/3.2/topics/db/queries/#spanning-multi-valued-relationships

Use sm breakpoint, same that is used in the filters form

update deps

See merge request !1879

https://docs.djangoproject.com/en/3.2/releases/3.2.1/
https://github.com/zostera/django-bootstrap4/blob/main/CHANGELOG.md
https://github.com/jazzband/django-ical/blob/master/CHANGES.rst

changelog for 0.59.0

See merge request !1872

Translations

See merge request !1873

Fix: only count appointments of participating subjects in progress view

See merge request !1874

Remove invited count from execution header

See merge request !1875

notify study members on sessions_end

See merge request !1869