Resolve "Prepare for Django 4.0"

Closes #563

See merge request !808

Resolve "DataCheckResult uses invalid attribute name `check`"

Closes #768

See merge request !1128

Resolve "Missing package spdx-license-list"

Closes #767

See merge request !1127

This also bumps Django to 4.1 because this undocumented API was
changed in a minor release…

# Conflicts:
#   CHANGELOG.rst

Mark code-entered invitation accepted after signup

See merge request !1119

Update dependency django-colorfield to ^0.8.0

See merge request !1120

Update dependency django_select2 to v8

See merge request !1109

Sync AlekSIS groups to Django groups on registration of existing Persons

Closes #752

See merge request !1117

Resolve "TaskUserAssignment matching query does not exist."

Closes #747

See merge request !1118

# Conflicts:
#   CHANGELOG.rst

Fix invitations of existing persons with short name

Closes #689

See merge request !1114