wiki web_ui: finally call `IWikiPageManipulator.prepare_wiki_page(req, page, fields)`.
This method was: ''Not currently called, but should be provided for future compatibility.''
Now we use it to ''Validate a wiki page before rendering it''. The `fields` can be used to manipulate the `text` before rendering.
Closes #9096.
Authored by: cboos 2010-03-22
Parent: [r9389]
Child: [r9391]
