(bug 42064) Call EditFilterMergedContent from EditEntity.

This is a partial fix to bug 42064, triggering EditFilterMergedContent from
EditEntity, so AbuseFilter and the like have a chance to filter edits
performed on Wikibase entities.

To do this, an IContextSource needs to be provided to the EditEntity instance.

This change requires the core change I99a19c93	to work correctly.

Change-Id: I5f5b4200f8a4db0e34790ad0c704ec9e17aff1f1
Merge "(bug 42064) Call EditFilterMergedContent from EditEntity."