Make base url of preview.html dynamic

Authored by Florian on May 19 2017, 5:07 PM.

Description

Make base url of preview.html dynamic

Like the changed for index.html, the preview.html base url is not
hardcoded to wikipedia.org anymore, but set via JS from the WikiSite which
is in use for the edit preview.

Bug: T165789
Change-Id: I95510a6aa7121362d5e3f860ed161b23452204ae