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/models/proposal_notification_spec.rb

@@ -43,7 +43,6 @@ describe ProposalNotification do
   end
 
   describe "minimum interval between notifications" do
-
     before do
       Setting[:proposal_notification_minimum_interval_in_days] = 3
     end
@@ -71,36 +70,29 @@ describe ProposalNotification do
 
       expect(notification1).to be_valid
     end
-
   end
 
   describe "notifications in-app" do
-
     let(:notifiable) { create(model_name(ProposalNotification)) }
     let(:proposal) { notifiable.proposal }
 
     describe "#notification_title" do
-
       it "returns the proposal title" do
         notification = create(:notification, notifiable: notifiable)
 
         expect(notification.notifiable_title).to eq notifiable.proposal.title
       end
-
     end
 
     describe "#notification_action" do
-
       it "returns the correct action" do
         notification = create(:notification, notifiable: notifiable)
 
         expect(notification.notifiable_action).to eq "proposal_notification"
       end
-
     end
 
     describe "notifiable_available?" do
-
       it "returns true when the proposal is available" do
         notification = create(:notification, notifiable: notifiable)
 
@@ -114,11 +106,9 @@ describe ProposalNotification do
 
         expect(notification.notifiable_available?).to be(false)
       end
-
     end
 
     describe "check_availability" do
-
       it "returns true if the resource is present, not hidden, nor retired" do
         notification = create(:notification, notifiable: notifiable)
 
@@ -147,7 +137,6 @@ describe ProposalNotification do
           retired_reason: "unfeasible")
         expect(notification.check_availability(proposal)).to be(false)
       end
-
     end
 
     describe "#moderate_system_email" do