Revert "Revert "Use WikiPage::prepareContentForEdit in SpamBlacklistHooks::filterMergedContent""

Ie71ebdeb should fix the bug in Wikibase that prompted the original
reversion.

This reverts commit 2745442aec0437d719bfad3b7a81580d2bf10fa8.

Change-Id: I085996669db7e0fcbf839b8d38d020c8d2e09220