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

lib/evaluation_comment_email.rb

@@ -22,6 +22,7 @@ class EvaluationCommentEmail
   end
 
   private
+
     def related_users
       return [] if comment.commentable.nil?
       comment.commentable