improved UI while simplifying AJAX form handling

moving form validation errors into the respective form puts the
respective messages in proper context

the slight increase in server-side complexity allows for simpler APIs
and less awareness on the client side

note, however, that pjax operations do not update global messages -
also, only the respective form's contents are updated, thus any change
in the form itself (e.g. `action` in multi-step workflows) would be
ignored

this reverts the previous commit
(6873b3fee5fd565799a9ab85b951a3d05219aa00)