API: Add iiprop=canonicaltitle

With recent changes to the filerepo backends, prop=imageinfo will now
return information for the target file when file redirects are queried.
To make this more transparent to clients, add an iiprop to return the
canonical title for the imageinfo data.

Change-Id: I62e368ffe09512f1af6677994f1ccca776b97651
Merge "API: Add iiprop=canonicaltitle"