This reverts commit 047bccfac1.
It created merge conflicts with a bunch of patches that were days old and about to be merged. Instead of rebasing 10 patches, let's revert this one, finish merging the other ones and just rebase and fix this one. I'll re-introduce the patch in a bit.
Change-Id: Ib495755b0ab4bfa5fdf5590b1271792862a47d4b
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