Benefit name must be unique

In response to this error:

```
  File "/srv/staging-pycon.python.org/shared/env/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 115, in get_response
    response = callback(request, *callback_args, **callback_kwargs)

  File "/srv/staging-pycon.python.org/shared/env/local/lib/python2.7/site-packages/django/contrib/admin/options.py", line 372, in wrapper
    return self.admin_site.admin_view(view)(*args, **kwargs)

  File "/srv/staging-pycon.python.org/shared/env/local/lib/python2.7/site-packages/django/utils/decorators.py", line 91, in _wrapped_view
    response = view_func(request, *args, **kwargs)

  File "/srv/staging-pycon.python.org/shared/env/local/lib/python2.7/site-packages/django/views/decorators/cache.py", line 89, in _wrapped_view_func
    response = view_func(request, *args, **kwargs)

  File "/srv/staging-pycon.python.org/shared/env/local/lib/python2.7/site-packages/django/contrib/admin/sites.py", line 202, in inner
    return view(request, *args, **kwargs)

  File "/srv/staging-pycon.python.org/shared/env/local/lib/python2.7/site-packages/django/utils/decorators.py", line 25, in _wrapper
    return bound_func(*args, **kwargs)

  File "/srv/staging-pycon.python.org/shared/env/local/lib/python2.7/site-packages/django/utils/decorators.py", line 91, in _wrapped_view
    response = view_func(request, *args, **kwargs)

  File "/srv/staging-pycon.python.org/shared/env/local/lib/python2.7/site-packages/django/utils/decorators.py", line 21, in bound_func
    return func(self, *args2, **kwargs2)

  File "/srv/staging-pycon.python.org/shared/env/local/lib/python2.7/site-packages/django/db/transaction.py", line 223, in inner
    return func(*args, **kwargs)

  File "/srv/staging-pycon.python.org/shared/env/local/lib/python2.7/site-packages/django/contrib/admin/options.py", line 1008, in add_view
    self.save_related(request, form, formsets, False)

  File "/srv/staging-pycon.python.org/releases/23e3028bd879deea9a0d0e3f1f084db8b700bdf1/pycon/sponsorship/admin.py", line 95, in save_related
    obj.save()

  File "/srv/staging-pycon.python.org/releases/23e3028bd879deea9a0d0e3f1f084db8b700bdf1/pycon/sponsorship/models.py", line 111, in save
    setattr(self, field_name, self.benefit_is_complete(benefit_name))

  File "/srv/staging-pycon.python.org/releases/23e3028bd879deea9a0d0e3f1f084db8b700bdf1/pycon/sponsorship/models.py", line 224, in benefit_is_complete
    benefit = self.sponsor_benefits.get(benefit__name=name)

  File "/srv/staging-pycon.python.org/shared/env/local/lib/python2.7/site-packages/django/db/models/manager.py", line 143, in get
    return self.get_query_set().get(*args, **kwargs)

  File "/srv/staging-pycon.python.org/shared/env/local/lib/python2.7/site-packages/django/db/models/query.py", line 407, in get
    (self.model._meta.object_name, num))

MultipleObjectsReturned: get() returned more than one SponsorBenefit -- it returned 2!
```