(bug 43775) Provide text for filtering to AbuseFilter.

This change also fixed EditEntity to provide more accurate
context. Specifically, EditEntity now makes sure to set the
correct Title and WikiPage objects in the Context passed
to the EditFilterMergedContent hook.

Change-Id: I649fb2e069871e1cc66c31bae5b88cff19deae82
Merge "(bug 43775) Provide text for filtering to AbuseFilter."