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* BI: renamed Notice.user to Notice.recipient
* BI: renamed {{ user }} context variable in notification templates to
{{ recipient }}
* BI: added nullable Notice.sender and modified send_now and queue to take
an optional sender
* added received and sent methods taking a User instance to Notice.objects
* New default behavior: single notice view now marks unseen notices as seen
* no longer optionally depend on mailer; use django.core.mail.send_mail and
we now encourge use of Django 1.2+ for mailer support
* notifications are not sent to inactive users
* users which do not exist when sending notification are now ignored
* BI: split settings part of notices view to its own view notice_settings