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/notification_spec.rb

@@ -1,11 +1,9 @@
 require "rails_helper"
 
 describe Notification do
-
   let(:notification) { build(:notification) }
 
   context "validations" do
-
     it "is valid" do
       expect(notification).to be_valid
     end
@@ -14,11 +12,9 @@ describe Notification do
       notification.user = nil
       expect(notification).not_to be_valid
     end
-
   end
 
   context "scopes" do
-
     describe "#read" do
       it "returns only read notifications" do
         read_notification1 = create(:notification, :read)
@@ -56,7 +52,6 @@ describe Notification do
         Notification.for_render
       end
     end
-
   end
 
   describe "#mark_as_read" do
@@ -172,7 +167,5 @@ describe Notification do
 
       expect(notification.notifiable_action).to eq "replies_to"
     end
-
   end
-
 end