Fix display of form validation errors in config forms.

The config forms code wasn't allowing errors to be shown on the forms,
and had other issues that prevented proper error reporting:

1. We didn't implement a post() function on the view, meaning that
   TemplateView would default to a Method Not Allowed.

2. We weren't replacing the form instance within the page, resulting in
   a clean form being used, free of errors.

3. We weren't showing non-field errors on the form.

This fixes these cases so that form validation errors will be shown
correctly.

It also moves all the POST handling into the new post() function, where
it really belongs.

Reviewed at https://reviews.reviewboard.org/r/6076/