Form: prevalidation form tuning

[agjohnson]

A few notes to come back to for tuning on this pattern:

• Turn the prevalidate form into a mixin, so that the base form can be forms.Form or forms.ModelForm without struggling with any conflicts. Maybe this isn't necessary.
• is_disabled should reference self.errors I think. This would populate the prevalidation errors if form.errors wasn't called first. The use case here is when displaying the form, we might use <form {% if form.is_disabled %}inert{% endif %}> before we actually call {{ form | crispy }}. The crispy filter calls form.errors which populates all the validation errors.
• Project: create forms are still usable by keyboard when disabled ext-theme#279 -- a few options here are <form inert>, disabling all of the fields in the Form class, or wrapping with .ui.disabled.segment instead. There is no .ui.disabled.form unfortunately.
• Revisit disabling forms pattern in Add Pull Request builds page to settings #10656 and related PR