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/admin_wysiwyg_sanitizer_spec.rb

@@ -4,7 +4,6 @@ describe AdminWYSIWYGSanitizer do
   let(:sanitizer) { AdminWYSIWYGSanitizer.new }
 
   describe "#sanitize" do
-
     it "allows images" do
       html = 'Dangerous<img src="/smile.png" alt="Smile" style="width: 10px;"> image'
       expect(sanitizer.sanitize(html)).to eq(html)
@@ -28,6 +27,5 @@ describe AdminWYSIWYGSanitizer do
               </table>'
       expect(sanitizer.sanitize(html)).to eq(html)
     end
-
   end
 end