Add ability to disable Rankings page

[dsevillamartin]

From ReFlar/gamification#49

At the moment, there is a rankings page that ranks users by the number of points that they have acquired on the forum. This is assumed to be a default desired feature of the gamification extension. I would appreciate it if you added the ability to disable this Rankings Page from the admin panel.

[Ralkage]

This was added to previous release and can be configured in the Admin area under the Gamification permissions. The label for this permission is called "See ranking page".