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/installation_controller.rb

@@ -1,5 +1,4 @@
 class InstallationController < ApplicationController
-
   skip_authorization_check
 
   def details
@@ -19,5 +18,4 @@ class InstallationController < ApplicationController
     def settings_feature_flags
       Setting.where("key LIKE 'feature.%'").each_with_object({}) { |x, n| n[x.key.remove("feature.")] = x.value }
     end
-
 end