ZanataMT - Further work

Attribution

• embed base64 attribution image into returned page (after header node)

• allow caller to specify attribution to be inserted inline or not

Move sourceLocale into Article DTO, also translationLocale (only for output; nullable)

Include backendId into Article DTO (only for output; nullable)

Remove backendId from endpoint URL, use MS by default (controlled by server)

Generate secured identity, api key for basic security check (system properties)