Mark invalid fields with .is-invalid instead of has-danger for Bootstrap 4 (beta)

[donv]

Bootstrap 4 beta is now out, promising no more breaking changes. Maybe make V4 the master?

Anyway, form fields should now be marked with .is-invalid or .is-valid depending on status.

The help text should be marked with .invalid-feedback.

Reference: https://getbootstrap.com/docs/4.0/components/forms/

[mattbrictson]

@donv can you confirm this issue has been fixed via #370?

[donv]

Indeed, this has been fixed!