Fix links on the '/features/security' page
Fix links on the '/features/security' page

- Changed the 'contact us' link to point to our support form, not the
  Notify homepage
- Updated the link to the details about CHECK based testing, since the
  site we were linking to no longer exists.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin