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/features/help_page_spec.rb

@@ -1,9 +1,7 @@
 require "rails_helper"
 
 describe "Help page" do
-
   context "Index" do
-
     scenario "Help menu and page is visible if feature is enabled" do
       Setting["feature.help_page"] = true