Removed extra fieldset in add_form.html

Issue #37 in master branch

removed fieldset that displayed username/password/verify password fields within the after_fields_sets block
because now in change_form.html (block field_sets), which add_form.html overrides, the username, password etc
fields are being displayed.  Thus on an initial add of a user, two sets of fields were being displayed and
one was unusable as well.

add_form.html now merely adds an explanation to the user why only username,etc are settable
Also hide Mezzanine's site permissions field for for consistency.