[web/views] Stop mishandling the fieldset name in the default form renderer While not really documented, it is generally assumed that: * the fieldset is a user-visible string (much like a title), * it will be translated by whichever piece of code uses it. So: * fix AutomaticEntityForm to generate untranslated fieldset names, * fix FormRenderer to properly translate the fieldset name before rendering it, * stop using the field set (which may contain whitespace) as a CSS class name (the feature was unusable, so just drop it), * properly handle the colon as part of the translated fieldset name Closes #4881298