Replace generic String "type" with more specific "HST-Type" for container and container-item metadata attributes

For the new channel-manager we expose page metadata as a JSON HTML-comment. To improve robustness of the comment-parsing on the client, the "type" attribute (which is used to validate if the data in a comment is actually a HST JSON metadata object) is renamed to "HST-Type". This still needs to be done for HST containers and container-items in order to keep the comment-parsing code generic. See HSTTWO-3583 for the page metadata.