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/admin/system_emails_spec.rb

@@ -1,7 +1,6 @@
 require "rails_helper"
 
 describe "System Emails" do
-
   let(:admin) { create(:administrator) }
 
   before do
@@ -9,7 +8,6 @@ describe "System Emails" do
   end
 
   context "Index" do
-
     let(:system_emails_with_preview) { %w[proposal_notification_digest] }
     let(:system_emails) do
       %w[proposal_notification_digest budget_investment_created budget_investment_selected
@@ -19,7 +17,6 @@ describe "System Emails" do
     end
 
     context "System emails" do
-
       scenario "have 'View' button" do
         visit admin_system_emails_path
 
@@ -29,11 +26,9 @@ describe "System Emails" do
           end
         end
       end
-
     end
 
     context "System emails with preview" do
-
       scenario "have 'Preview Pending' and 'Send pending' buttons" do
         visit admin_system_emails_path
 
@@ -49,11 +44,9 @@ describe "System Emails" do
           end
         end
       end
-
     end
 
     context "System emails with info" do
-
       scenario "have information about how to edit the email templates" do
         visit admin_system_emails_path
 
@@ -69,11 +62,9 @@ describe "System Emails" do
         end
       end
     end
-
   end
 
   context "View" do
-
     let(:user)    { create(:user, :level_two, username: "John Doe") }
     let(:budget)  { create(:budget, name: "Budget for 2019") }
     let(:heading) { create(:budget_heading, budget: budget) }
@@ -267,7 +258,6 @@ describe "System Emails" do
       expect(page).to have_link "Cleaner city",
         href: admin_budget_budget_investment_url(investment.budget, investment, anchor: "comments")
     end
-
   end
 
   context "Preview Pending" do
@@ -336,5 +326,4 @@ describe "System Emails" do
       expect(page).to have_content("Pending notifications sent succesfully")
     end
   end
-
 end