refs #242

The FormGroup’s save-indicator would jump around when used in
combination with the FormTable. We now have a more tight coupling for
these components so that the FormTable can influence the FormGroup’s
rendering.

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.

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

refs #140