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/email_digest.rb

@@ -1,5 +1,4 @@
 class EmailDigest
-
   attr_accessor :user
 
   def initialize(user)
@@ -29,5 +28,4 @@ class EmailDigest
   def valid_email?
     user.email.present? && user.email.match(/\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\Z/i)
   end
-
 end