Don't expect messages to always have text

Some messages are notices that don't carry any text (responses to forum invitations for example).

```
Traceback (most recent call last):
  File "/usr/share/briar-gtk/briar_gtk/containers/private_chat.py", line 101, in _add_message
    message_widget = PrivateMessageWidget(self._contact_name, message)
  File "/usr/share/briar-gtk/briar_gtk/widgets/private_message.py", line 18, in __init__
    self._setup_view(contact_name, message)
  File "/usr/share/briar-gtk/briar_gtk/widgets/private_message.py", line 34, in _setup_view
    message["text"])
KeyError: 'text'
```

Fixes #56.
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