Call new Content::prepareSave from WikiPage::doEditContent.

Call new Content::prepareSave from WikiPage::doEditContent.

Content::prepareSave() is called  by WikiPage::doEditContent() inside the
transaction bracket for saving the new content, but before anything is actually
saved. This allows the content to first be normalized and checked against the
current database content; it also allows for some data to be written to the
database up front.

Change-Id: I78946ea5ec34dce08068692e2d77a8a0b7c11777