Accessing the help page while not logged in returns a 500 response

That looks like:

```python
File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/django/template/base.py", line 844, in render_node
    return node.render(context)

  File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/django/template/loader_tags.py", line 63, in render
    result = block.nodelist.render(context)

  File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/django/template/base.py", line 830, in render
    bit = self.render_node(node, context)

  File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/django/template/base.py", line 844, in render_node
    return node.render(context)

  File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/django/template/base.py", line 1114, in render
    return func(*resolved_args, **resolved_kwargs)

  File "/var/praekelt/vumi-go/go/billing/templatetags/billing_tags.py", line 14, in credit_balance
    account = user.account_set.all()[0]

  File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/django/utils/functional.py", line 205, in inner
    return func(self._wrapped, *args)

AttributeError: 'AnonymousUser' object has no attribute 'account_set'
```