Update message

Signed-off-by: Simon Luijk <simon@simonluijk.com>
Add CreateModelMixin

This is almost identical to the mixin provided by django-rest-framework.
The only two changes have notes above them. These two changes allow for
more control over which serializer is used after creating an object.

Signed-off-by: Simon Luijk <simon@simonluijk.com>
Remove user serializer mixins.

UniqueEmailUserSerializerMixin is not needed as newer versions of
django-rest-framework handle this case when unique is set on a model field.

PasswordSerializerMixin is also not the best solution now we have the
CreateModelMixin.

Signed-off-by: Simon Luijk <simon@simonluijk.com>