Translations update from Weblate

See merge request !1301

Currently translated at 100.0% (219 of 219 strings)

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

Manage holidays

See merge request !1261

Translations update from Weblate

See merge request !1279

Currently translated at 100.0% (640 of 640 strings)

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

Translations update from Weblate

See merge request !1297

Currently translated at 100.0% (212 of 212 strings)

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

Update translation files

See merge request !1296

[Docs] Add missing package

See merge request !1295

Calendar events and iCal feeds

See merge request !1148

Update dependency graphene-django to >=3.0.0, <=3.1.3

See merge request !1294

Merge branch 'calendar-object-feeds' of edugit.org:AlekSIS/official/AlekSIS-Core into calendar-object-feeds

Resolve "Provide URL representation for every object"

Closes #871

See merge request !1268

Commit fa4df9e5 had unresolved merge
conflicts. They are merged now. This also adds a newline the
aleksolotl missed.

Resolve "'fields_for_model' is not defined in ExtensibleForm"

Closes #887

See merge request !1292