[JRO] Move responsibility of getting user board preferences into messageboard class

There were several spots with some code smells pointing towards the need to tell, not ask. ApplicationController had a method passed down to inheriting classes that provided a user's messageboard preferences. A check for existence of user could be pushed off to a null object if the query returned nil.

```
messageboard.preferences_for(user) => preference from db
messageboard.preferences_for(nil) = null preference
```