Add in a 'TestCanonicalRedirect' hook to be called when about to make a redirect...

Add in a 'TestCanonicalRedirect' hook to be called when about to make a redirect to the canonical page form of a regular page view. Some extensions such as MobileProfile may need to modify this logic to prevent incorrect redirects, such as on alternate en.m.wikipedia.org domain, where we'd want to either skip it or rewrite to the alternate domain.

Support for resolving bug 31012.