Make parent group field not required.

Closes #103

See merge request Teckids/BiscuIT/BiscuIT-ng!57

Currently translated at 100.0% (110 of 110 strings)

Translation: BiscuIT School Information System/BiscuIT-ng
Translate-URL: https://translate.edugit.org/projects/biscuit-sis/biscuit-ng/de/

Make official name of school translatable. Closes #95.

Closes #95

See merge request Teckids/BiscuIT/BiscuIT-ng!54

Currently translated at 100.0% (109 of 109 strings)

Translation: BiscuIT School Information System/BiscuIT-ng
Translate-URL: https://translate.edugit.org/projects/biscuit-sis/biscuit-ng/de/

Use btn-dark everywhere. Closes #94.

Closes #94

See merge request Teckids/BiscuIT/BiscuIT-ng!53

Use bootstrap buttons instead of input tag.

See merge request Teckids/BiscuIT/BiscuIT-ng!52

This is wrong from a Python point of view, but the default in the
Django Template Language.

Currently translated at 100.0% (108 of 108 strings)

Translation: BiscuIT School Information System/BiscuIT-ng
Translate-URL: https://translate.edugit.org/projects/biscuit-sis/biscuit-ng/de/