Translations update from Weblate

See merge request !1002

Currently translated at 100.0% (663 of 663 strings)

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

Update translation files

See merge request !1001

Resolve "Account menu is duplicated due to merge errors"

Closes #680

See merge request !1000

Move entry to Unreleased section

See merge request !999

Resolve "Allow to disable health check failure mails"

Closes #527

See merge request !763

Resolve "Weekdays not translated"

Closes #655

See merge request !998

Resolve "Icons are loaded from external servers"

Closes #678

See merge request !997

Improve actionform

See merge request !996

Resolve "Fix menu button being displayed twice on small screens"

Closes #677

See merge request !995

Prepare release 2.8.1

See merge request !994

Resolve "Allow overriding settings in trusted apps"

Closes #675

See merge request !993

Release 2.8

See merge request !992

Resolve "Change material icon set"

Closes #258

See merge request !686