Resolve "Backup health check broken when using S3"

Closes #557

See merge request !800

Limit Celery dependency to <5.2 for now

See merge request !801

This code was buggy for some storages, and useless anyway: If the
storage base directory did not exist, retrieving the backup list would
have failed altogether, and the check returned error.

Resolve "[OAuth] Application form missing algorithm field"

Closes #555

See merge request !798

Various OAuth fixes

Closes #550, #551, #552, and #554

See merge request !796

Merge branch '554-typeerror-unsupported-operand-type-s-for-set-and-tuple' into 550-oauth-groups-feature-missing-registered-scope

Add trademark hints

See merge request !787

Closes #552

Closes #551.

Resolve "[Docker] startup script continues after failed migrations"

Closes #549

See merge request !795

Bump dev version to 2.3.dev0

See merge request !794

Prepare release 2.2

See merge request !793

Translations update from Weblate

See merge request !792

Currently translated at 100.0% (603 of 603 strings)

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

Update translation files

See merge request !791

Support (icon) tabs in top navbar

See merge request !789