Instead load it via mw.loader.using
We retain the module name ext.popups as this will be present
in cached HTML, however now it will load the bulk of the code
inside ext.popups.main
Bug: T176211
Change-Id: Ibe212721807d3698dc45ef46b2dbde15ca9d2f70
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