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.
Hide the site perms field in the user add form without breaking the form submission.
Add blocks submit_buttons_top and submit_buttons_bottom

Backport of the same commit in django-grappelli.
Remove a ton of unused templates.
Version bump to 0.3.13
Update bulk-deletion confirmation template

  - Don't show confirmation form when deletion is not allowed
  - Correct the grammar