Minor improvements to the server roles model

- Some bugfixes where we should've been using .server_roles for roles selecting/deselection
- Added a property for inspecting the current reported server roles
- Stripped the 'server_roles_' prefix off of role names, that's unique to this page and not part of the actual role names
Merge pull request #73 from seandst/add-roles-fixture

Minor improvements to the server roles model