Add SiteMapId to Channel resource

The old channel manager extracted the channel's siteMapId for retrieving the channel's sitemap (Pages dialog) from the preview's response headers. For the new channel manager, this is unfortunate, because

1) the siteMapId pertains to the current channel rather than the current page, and
2) using a value from the page metadata introduces an additional, undesirable dependency between the interactions between the preview-site and the back-end, and the channel manager and the back-end.

Therefore we're going to add a channel's siteMapId to the channel's metadata.