Change `from django.utils.timezone import now` to
`from django.utils import timezone`.
This is both because now() is ambiguous (could be datetime.datetime.now),
and more importantly to make it easier to write a lint rule against
datetime.datetime.now().
When we push a device token, we want to clean out any other user's
tokens on the device, but not the current user's. We were wiping
away our own token, if it existed, before creating it again. This
was probably never a user-facing problem; it just made for dead code
and a little unnecessary DB churn. By excluding the current user
from the delete() call, we exercise the update path in our tests now,
so we have 100% coverage.