Update "comment" 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.

We also adapted the existing generic text by adding a description of
the related notification.

spec/system/admin/system_emails_spec.rb

@@ -155,6 +155,9 @@ describe "System Emails" do
       expect(page).to have_content comment.body
 
       expect(page).to have_link "Let's do...", href: debate_url(debate, host: app_host)
+      expect(page).to have_link("Notifications",
+                                href: edit_subscriptions_url(token: user.subscriptions_token,
+                                                             host: app_host))
     end
 
     scenario "#reply" do