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/helpers/proposals_helper_spec.rb

@@ -1,7 +1,6 @@
 require "rails_helper"
 
 describe ProposalsHelper do
-
   describe "#progress_bar_percentage" do
     it "is 0 if no votes" do
       proposal = create(:proposal)
@@ -39,7 +38,5 @@ describe ProposalsHelper do
       proposal = create(:proposal, cached_votes_up: Proposal.votes_needed_for_success * 2)
       expect(supports_percentage(proposal)).to eq "100%"
     end
-
   end
-
 end