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/features/stats_spec.rb

@@ -1,9 +1,7 @@
 require "rails_helper"
 
 describe "Stats" do
-
   context "Summary" do
-
     scenario "General" do
       create(:debate)
       2.times { create(:proposal) }
@@ -41,7 +39,5 @@ describe "Stats" do
       expect(page).to have_content "Verified users 3"
       expect(page).to have_content "Unverified users 2"
     end
-
   end
-
 end