Use ApiWikibase::loadEntityContent

Instead of using a method in claimModificationHelper
use the one that already exists in the APIBase!

Also make sure that all users of the method also
load the correct / required baserevid...

Change-Id: I0b5958d3542bfb61b0639955a1e9ef54d7b186aa
Merge "Use ApiWikibase::loadEntityContent"