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/proposal_notifications_spec.rb

@@ -1,7 +1,6 @@
 require "rails_helper"
 
 describe "Proposal Notifications" do
-
   scenario "Send a notification" do
     author = create(:user, :with_proposal)
 
@@ -156,7 +155,6 @@ describe "Proposal Notifications" do
   end
 
   context "Permissions" do
-
     scenario "Link to send the message" do
       author = create(:user)
       proposal = create(:proposal, author: author)
@@ -188,11 +186,9 @@ describe "Proposal Notifications" do
       expect(page).to have_current_path(root_path)
       expect(page).to have_content("You do not have permission to carry out the action")
     end
-
   end
 
   context "In-app notifications from the proposal's author" do
-
     scenario "Voters should receive a notification", :js do
       author = create(:user)
       proposal = create(:proposal, author: author)
@@ -342,7 +338,6 @@ describe "Proposal Notifications" do
     end
 
     context "Group notifications" do
-
       before do
         Setting[:proposal_notification_minimum_interval_in_days] = 0
       end
@@ -375,7 +370,6 @@ describe "Proposal Notifications" do
         expect(page).to have_content "There is one new notification on #{proposal.title}", count: 3
       end
     end
-
   end
 
   scenario "Error messages" do
@@ -391,7 +385,6 @@ describe "Proposal Notifications" do
   end
 
   context "Limits" do
-
     scenario "Cannot send more than one notification within established interval" do
       author = create(:user)
       proposal = create(:proposal, author: author)
@@ -439,7 +432,5 @@ describe "Proposal Notifications" do
 
       travel_back
     end
-
   end
-
 end