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

spec/models/setting_spec.rb

@@ -238,7 +238,6 @@ describe Setting do
   end
 
   describe ".force_presence_date_of_birth?" do
-
     it "return false when feature remote_census is not active" do
       Setting["feature.remote_census"] = false
 
@@ -261,7 +260,6 @@ describe Setting do
   end
 
   describe ".force_presence_postal_code?" do
-
     it "return false when feature remote_census is not active" do
       Setting["feature.remote_census"] = false