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/notifications_spec.rb

@@ -1,7 +1,6 @@
 require "rails_helper"
 
 describe "Notifications" do
-
   let(:user) { create :user }
 
   before do
@@ -227,7 +226,6 @@ describe "Notifications" do
       expect(Delayed::Job.second.run_at.change(usec: 0)).to eq(second_batch_run_at)
       expect(Delayed::Job.third.run_at.change(usec: 0)).to eq(third_batch_run_at)
     end
-
   end
 
   def remove_users_without_pending_notifications
@@ -239,5 +237,4 @@ describe "Notifications" do
       user.notifications.not_emailed.where(notifiable_type: "ProposalNotification").blank?
     end
   end
-
 end