Resolve "Replace dev.sh with tox environments"

Closes #266

See merge request !730

Update changelog from 2.0

See merge request !756

(cherry picked from commit ec156077)

Resolve "Caching issues"

Closes #483

See merge request !747

Translations update from Weblate

See merge request !726

Currently translated at 100.0% (581 of 581 strings)

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

Currently translated at 100.0% (581 of 581 strings)

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

Translations update from Weblate

See merge request !754

Currently translated at 100.0% (581 of 581 strings)

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

Currently translated at 100.0% (4 of 4 strings)

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

Update translation files

See merge request !753

Resolve "person_accounts view hangs while rendering template"

Closes #497

See merge request !750

Merge branch '509-users-are-able-to-change-the-linked-user-but-not-the-supposed-fields' into 'master'

Resolve "Users are able to change the linked user, but not the supposed fields"

Closes #509

See merge request !752

Fix order in changelog

See merge request !749

Fix page title in impersonate template

Closes #522

See merge request !748

Introduce an ExtensiblePolymorphicModel

See merge request !701

Close #522