Set the real message.id in the response instead of the content hash

Introduce the new field "content_id" and use it for the same purpose as
the "id" field was used: to detect if a dismissed message should be
displayed again. This behaviour is described in
I2ccaaa55ac549cb37eed851f8ed40f4d5f345f72

Change-Id: Ifd67230ea09884e0f31b140e2e2ccf550f0534b5