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/lib/reply_email_spec.rb

@@ -33,7 +33,6 @@ describe ReplyEmail do
   end
 
   describe "#can_be_sent?" do
-
     it "returns true if comment and recipient exist" do
       expect(reply_email.can_be_sent?).to be true
     end
@@ -49,7 +48,5 @@ describe ReplyEmail do
 
       expect(reply_email.can_be_sent?).to be false
     end
-
   end
-
 end