Resolve "AttributeError 'InvitePersonByID' object has no attribute 'ge..."

Closes #644

See merge request !954

[Docs] Add missing image

See merge request !959

[Docs] Update environment docs for developing own apps

See merge request !958

Bump version to 2.7.3

See merge request !952

Resolve "All icons referred in the manifest have purpose: "maskable" set"

See merge request AlekSIS/official/AlekSIS-Core!951

Merge branch '642-fwd-schoolapps-celery-task-aleksis-core-tasks-clear_oauth_tokens-fehlgeschlagen-ticket-751413' into 'master'

Resolve "Fwd: [SchoolApps] Celery-Task aleksis.core.tasks.clear_oauth_tokens fehlgeschlagen! [Ticket#751413]"

Closes #642

See merge request AlekSIS/official/AlekSIS-Core!950

[Docs] Further improvements

Closes #614, #617, and #625

See merge request AlekSIS/official/AlekSIS-Core!910

[Docs] Data checks

See merge request AlekSIS/official/AlekSIS-Core!926

[Docs] Registration and user invitations

See merge request !922

Resolve "'FieldInstanceTracker' object has no attribute 'saved_data'"

Closes #639

See merge request AlekSIS/official/AlekSIS-Core!948

Resolve "Migration 0035_preference_model_unique still broken"

Closes #638

See merge request AlekSIS/official/AlekSIS-Core!949

django-model-utils' documentation states that using the relation is not stable in all situations.

Bump version to 2.7.2 and update changelog

See merge request !946

Translations update from Weblate

See merge request !930

Currently translated at 99.8% (645 of 646 strings)

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

Update translation files

See merge request !945

Use separate database connection for Celery results

See merge request !943

This ensures Celery progress is always updated and queried outside a
transaction.