Don't generate an extra form and don't fill in values

The extra form has to be hidden, and the values confuse things and
appear in hidden forms.
show indicator errors on page for now
refactor javascript logic

- Only expect the empty __prefix__ indicator formset - any other formsets
  in the html contain data
- if there are no formsets, create one empty one
- when copying a formset, clear all values in it
- when copying the value from textarea replacement to the matching
  hidden <input>, do it in a way that works however they were set up
- always create the matching hidden <input> for new textarea
  replacements