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

app/helpers/comments_helper.rb

@@ -1,5 +1,4 @@
 module CommentsHelper
-
   def comment_tree_title_text(commentable)
     if commentable.class == Legislation::Question
       t("legislation.questions.comments.comments_title")
@@ -83,5 +82,4 @@ module CommentsHelper
       t("comments.comments_closed")
     end
   end
-
 end