Enable user registration

[Daniele Viganò]

These modification are needed for an open user registration:

- /account/signup/ must be white-listed otherwise it cannot be seen by unauthenticated users
- REGISTRATION_OPEN must be set to True
- with ACCOUNT_ACTIVATION_DAYS you can define number of days in which account will be left active without a successful activation

Be careful, by default there's no admin validation mechanism on new accounts. New accounts are automatically enabled. They must be validated by users using link in the confirmation email within ACCOUNT_ACTIVATION_DAYS days. I don't like this approach. I would like to have the email validated before the account is activated, but this is the default (and only) way in GeoNode.

See: http://geonode.readthedocs.org/en/latest/tutorials/users/accounts/registration.html and https://django-registration.readthedocs.org/en/latest/default-backend.html

Also a working mailserver must be set.

[Daniele Viganò (daniele-vigano)]

PR: https://github.com/gem/oq-platform/pull/249

We still need to improve the way how users can register.