Implement default welcome message

- Allow Newsletter records to have blank description and welcome
- Add a setting to define a default welcome message
- Use that setting when sending a welcome (for now, until we decide
  how to handle subscribing to multiple newsletters at once)
- Add Newsletter.get_message() to return either the newsletter's own
  welcome message, or the default one
- Update fixture to remove placeholder descriptions and welcomes like
  'n/a'.

Bug 841426, 829278