(bug 42064) AbuseFilter + EditFilterMergedContent

This makes AbuseFilter use EditFilterMergedContent if support for
the ContentHandler infrastructure is present. This means living
without some nice bits of context, because EditFilterMergedContent
doesn't provide an EditPage object.

This requires core change I99a19c93 to work correctly.

Change-Id: Ibb9d4c9a36b8a199213958b920902e8006c71fe8
Use Status object to report filter results.

This changes the AbuseFilter to use Status objects for internal
error handling. This allows for more flexibility in passing error
messages back from hooks and avoids double escaping.

Change-Id: I72e1a6dd7dee19f889fc13b60456e9bfebd5e22b
Merge changes I72e1a6dd,Ibb9d4c9a

* changes:
  Use Status object to report filter results.
  (bug 42064) AbuseFilter + EditFilterMergedContent