Unify and improve email subjects for all emails

Some subjects had a prefix (e.g. [Instance]), some subjects were misleading (e.g. failed payment email had 'Order Confirmation', some can be clearer by adding information like order number, distributor or coordinator

spec/mailers/report_mailer_spec.rb

@@ -13,7 +13,7 @@ RSpec.describe ReportMailer do
     let(:blob) { ReportBlob.create_locally!("customers.csv", "report content") }
 
     it "notifies about a report" do
-      expect(email.subject).to eq "Report ready"
+      expect(email.subject).to eq "Report ready for download"
       expect(email.body).to have_content "Your report is ready for download."
     end
 

spec/mailers/shipment_mailer_spec.rb

@@ -35,9 +35,9 @@ RSpec.describe Spree::ShipmentMailer do
     }.not_to raise_error
   end
 
-  it "includes the distributor's name in the subject" do
+  it "includes the order number and distributor's name in the subject" do
     shipment_email = Spree::ShipmentMailer.shipped_email(shipment, delivery: true)
-    expect(shipment_email.subject).to include("#{distributor.name} Shipment Notification")
+    expect(shipment_email.subject).to include("Shipment notification of order #{shipment.order.number} at #{distributor.name}")
   end
 
   it "includes the distributor's name in the body" do
@@ -52,6 +52,6 @@ RSpec.describe Spree::ShipmentMailer do
 
   it "picked_up email has different subject" do
     shipment_email = Spree::ShipmentMailer.shipped_email(shipment, delivery: false)
-    expect(shipment_email.subject).to include("#{distributor.name} Pick up Notification")
+    expect(shipment_email.subject).to include("Pick up notification of order #{shipment.order.number} at #{distributor.name}")
   end
 end

spec/mailers/user_mailer_spec.rb

@@ -46,7 +46,7 @@ RSpec.describe Spree::UserMailer do
         email.deliver_now
 
         expect(ActionMailer::Base.deliveries.first.subject).to include(
-          "Please confirm your OFN account"
+          "Please confirm your account on OFN Demo Site"
         )
       end
     end