[JRO] Decorate messageboard and clean up the view

The _messageboard partial was leaning on some legacy helper methods that
looked a little hairy and needed cleaning up. The helpers being used
contained too many conditionals and were innefficient. Extracting those
responsibilities out to a decorator and making the view easier to read
cleans things up.

* Update gems
* Allow messagebord object or collection to be decorated with a `#decorate` method
* Introduce a NullTopic class to stand in for instances where a topic is not found
* Specs for decorator class