Add and apply rubocop rules for empty lines

We were very inconsistent regarding these rules.

Personally I prefer no empty lines around blocks, clases, etc... as
recommended by the Ruby style guide [1], and they're the default values
in rubocop, so those are the settings I'm applying.

The exception is the `private` access modifier, since we were leaving
empty lines around it most of the time. That's the default rubocop rule
as well. Personally I don't have a strong preference about this one.


[1] https://rubystyle.guide/#empty-lines-around-bodies

app/controllers/admin/valuator_groups_controller.rb

@@ -1,5 +1,4 @@
 class Admin::ValuatorGroupsController < Admin::BaseController
-
   def index
     @groups = ValuatorGroup.all.page(params[:page])
   end
@@ -48,5 +47,4 @@ class Admin::ValuatorGroupsController < Admin::BaseController
     def group_params
       params.require(:valuator_group).permit(:name)
     end
-
 end