Added support for hidden apps (bug 1019249, 1019251)

Well if we really want to change that code, I'd prefer:

```
updates = {
    'vip_app': self.cleaned_data.get('vip_app'),
    'priority_review': self.cleaned_data.get('priority_review')
}
if contact is not None:
    updates['mozilla_contact'] = contact
```

Or even (if the code underneath accepts empty `mozilla_contact`):

```
updates = {
    'mozilla_contact': self.cleaned_data.get('mozilla_contact'),
    'vip_app': self.cleaned_data.get('vip_app'),
    'priority_review': self.cleaned_data.get('priority_review')
}
```
(and in that case remove `contact = ...` line above)