Update "proposal" notification email to add unsubscribe link

We modified the link that previously redirected us to the "My content"
page to redirect us to the new page for managing subscriptions.

spec/system/admin/system_emails_spec.rb

@@ -92,6 +92,7 @@ describe "System Emails" do
                                 href: proposal_url(proposal_b, anchor: "tab-notifications", host: app_host))
       expect(page).to have_content("Proposal A Notification Body")
       expect(page).to have_content("Proposal B Notification Body")
+      expect(page).to have_link "Notifications"
     end
 
     scenario "#budget_investment_created" do