[0.2.X] Added send_mail to AbstractEmailUser for consistency with Django's AbstractUser.

Backport of 4bf520695b68f143d00b45bfd82553e172e06fd0 from master
[0.2.x] Give the ability to restrain which users can reset their password

Backport of 49115cfb29ce60dd88a63490f016739bdf7ef15c from master
[0.2.x] Update safe urls

Same bug as fusionbox/django-widgy#212

Backport of 75de4b618a1eaf8f1213fd87670c95d86fcb6864 from master