Fix PermissionsGroup validation always returning true, even though the
config was incorrectly setup.

Signed-off-by: Staartvin <mijnleraar@msn.com>
Fix message when no rank up was configured.

Signed-off-by: Staartvin <mijnleraar@msn.com>
Check if player has already completed this rank before.

Signed-off-by: Staartvin <mijnleraar@msn.com>
Track progress of the completed ranks.

Signed-off-by: Staartvin <mijnleraar@msn.com>
Update RequirementHandler to use UUID instead of player names. 
This reduces the load on the server, because we don't have to look up
all the names.

Signed-off-by: Staartvin <mijnleraar@msn.com>