No longer use livepreview-loading from core

It was no longer used in MediaWiki 1.24. Added it as a new message to
SemanticMediaWiki.

Also changed spaces to tabs in en.json.

Change-Id: I14448aecbbef2cfcfb5f47bd882046f0cc3c3ad1
Merge "No longer use livepreview-loading from core"