Role name method in Topic model

[skyegallup]

Topics are stored in the server's role list in the format Topic | <Topic.name>. This Topic | prefix has to be prepended (PREFIX + Topic.getName()) everywhere that mentorbot interacts with the server's role list. It would be better to create a new method, Topic.getRoleName(), that handles this within the model itself, so that the controller layer does not have to worry about this concatenation.

[skyegallup]

Blocked by #14 , since that commit is what adds Server.TOPIC_PREFIX. (On that note: shouldn't this constant be stored in Topic instead of Server?)