Use Select2 everywhere

Editing groups is a hassle, because editing the multi-select field is cumbersome (selecting is unergonomic, and accidentally remving members or owners is easy).

While at it, also fix this type of issue everywhere else.

changed milestone to %1.0a3

added part::frontend prio::2 labels

assigned to @debdolph

https://medium.com/@alfarhanzahedi/customizing-modelmultiplechoicefield-in-a-django-form-96e3ae7e1a07

Maybe we want something like this...

We could also use Select2:

https://github.com/applegrew/django-select2

changed title from Better group editor to Use Select2 everywhere

changed the description

mentioned in merge request !58 (merged)

created merge request !58 (merged) to address this issue

closed via merge request !58 (merged)

mentioned in commit d17fd4bb

reopened

unassigned @debdolph

added part::backend part::frontend prio::1 type::refactoring labels

assigned to @ZugBahnHof

assigned to @yuha and unassigned @ZugBahnHof

created merge request !362 (merged) to address this issue

mentioned in merge request !362 (merged)

Theoretically finished and working.

closed with merge request !362 (merged)

mentioned in commit 02d63ca2