Add a case reference to data breach form

Add a unique case reference to the data breach form to prevent incorrect
threading by GMail.

Requires mysociety/alaveteli#7967
Fixes #1774

lib/data_breach.rb

@@ -63,7 +63,8 @@ def data_breach(report, logged_in_user)
       mail(
         from: from,
         to: contact_from_name_and_email,
-        subject: _('New data breach report'),
+        subject: _('New data breach report [{{reference}}]',
+                   reference: case_reference('BR'))
       )
     end
   end

spec/integration/report_a_data_breach_spec.rb

@@ -36,7 +36,7 @@
     last_email = ActionMailer::Base.deliveries.last
     expect(last_email.from).to eq(['do-not-reply-to-this-address@localhost'])
     expect(last_email.to).to eq(['postmaster@localhost'])
-    expect(last_email.subject).to eq('New data breach report')
+    expect(last_email.subject).to match(/New data breach report \[BR\/.*\]/)
     expect(last_email.header["Reply-To"].value).to eq('test@example.com')
     expect(last_email.body).to include('URL: https://example.com')
     expect(last_email.body).to include('Special category or criminal offence data: Yes')