[#277] fix bug where attachments were stripped from hotline msgs

• SYMPTOM: when users send pictures in a hotline message, admins don't see them
• CAUSE: relayHotlineMessage was implemented by finishing with a call to notify (b/c that's how other functions that send messages to all admins work), but notify assumes it is only given a recipient and a messageBody string (b/c most of the time it is!). thus to conform with notify's signature, we strip out the rest of the fields of the SignaldOutgoingMessage returned from addHeaders and in so doing, drop the attachment from message
• FIX: don't use notify! just call signal.sendMessage

Closes #277 (closed)