Updated FormPreview to use form_hmac rather than the old security_hash function

This ought to have been done in [14218], but although the FormPreview class
was modified, and some tests were added, the crucial lines of code were not
changed (the 'FormPreview.security_hash' method), and tests for the new
behaviour were not added.  So it is being changed now.  Unlike some of the
other code in that changeset, this does not need to have a compatibility
fallback to cope with existing hashes, because the consequence of a failed
hash is minimal - the user is re-presented with the preview stage of the
form, which will then have a correct hash.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15952 bcc190cf-cafb-0310-a4f2-bffc1f526a37