Changes:
* Make the gateway handle missing pages, which are characterised by the
MediaWiki API response having both the missing property set to truthy
and the page not having any revisions.
* Add the preview-empty template and associated "mwe-popups-is-empty"
CSS class, which describe what an empty preview contains and how it
should look.
Supporting changes:
* Move the original preview template into the ext.popups module.
Bug: T151054
Change-Id: Ife75bf9c6bafdfe0a6cc3e20eea853b4ac8f951b
