Use RESTBase v1 API

This requires https://gerrit.wikimedia.org/r/214351 in core, which adds
support for emulating the RESTBase v1 API to the
ParsoidVirtualRESTService.

$wgVisualEditorPrefix is now deprecated. Parsoid is configured using
VRS (preferred) or $wgVisualEditorDomain, which defaults to
$wgCanonicalServer.

This bumps our MW-core dependency to 1.26wmf21.

Bug: T110780
Change-Id: I9175ac8db8310f0c9574c8272b5e39932557d57e