ujs: disable buttons on submit by default
ujs: spin for submitted forms and data-clear

if data-clear is set for a form tag the form will be reset once the form is submitted.
remove unused essential_initialization

appears to not be so essential in the end.
use ujs remote form for creating possibles
fix error message display for new possibilities
refactor: split spinner and text_spinner

using content_tag and tag instead of strings + html_safe