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/application_helper_spec.rb

@@ -1,7 +1,6 @@
 require "rails_helper"
 
 describe ApplicationHelper do
-
   describe "#author_of?" do
     it "is true if user is the author" do
       user = create(:user)
@@ -23,5 +22,4 @@ describe ApplicationHelper do
       expect(author_of?(proposal, nil)).to eq false
     end
   end
-
 end