Updating related name on policy.users, because 2 refs to users caused co...

Creator was conflicted with Users. 

Before fix:
```
In [1]: from teamwork.models import Policy

In [2]: policy = Policy.objects.all()[0]

In [3]: policy.users.all()
Out[3]: [<User: Joe-Analyst>]

In [4]: u = policy.users.all()[0]

In [5]: u.policy_set.all()
Out[5]: []
```

After fix:
```
In [1]: from teamwork.models import Policy

In [2]: policy = Policy.objects.all()[0]

In [3]: policy.users.all()
Out[3]: [<User: Joe-Analyst>]

In [4]: u = policy.users.all()[0]

In [5]: u.policy_set.all()
Out[5]: [<Policy: Policy(Exploit CAP Imagery)>, <Policy: Policy(Exploit CAP Imagery)>]
```

Please review tests associated with this. To me it looks like it was meant to be this way in the tests, but just in case, it should probably be checked.