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/comments_helper_spec.rb

@@ -11,9 +11,7 @@ require "rails_helper"
 #   end
 # end
 RSpec.describe CommentsHelper, type: :helper do
-
   describe "#user_level_class" do
-
     def comment_double(as_administrator: false, as_moderator: false, official: false)
       user = instance_double("User", official?: official, official_level: "Y")
       instance_double("Comment", as_administrator?: as_administrator, as_moderator?: as_moderator, user: user)