Bootstrap only displays form group errors if the .invalid-feedback
element is preceded by a .is-invalid element. That is an uncessary
restriction and we can simply enforce the errors to be displayed if it
is rendered.

Django rest framework would sometimes return objects like this

  { "email": ["Gib eine gültige E-Mail Adresse an."] }

which we didn’t properly handle.

refs #126

These form components still use bootstrap CSS classes but are part of
our effort to move away from bootstrap.

refs #140

Interfering with the default dialog close logic isn’t that smart because
it handles stuff like nested dialogs for us.

We need to find some other logic to close the dialog when someone clicks
on the backdrop, but that can be a task for a later time.

Using the same name as the native HTML dialog is a bad idea.

closes #139