When users visit a Phame post URI with an old blog ID, canonicalize the URI instead of 404'ing

Summary:
Fixes T13353. If you:

• Visit a blog post and save the URI.
• Move the blog post to a different blog.
• Revisit the old URI.

...we currently 404. We know what you're trying to do and should just redirect you to the new URI instead. We already do this if you visit a URI with a noncanonical slug.

Test Plan:

• Created post A.
• Copied the live URI.
• Moved it to a different blog.
• Visited the saved URI from the earlier step.
• Before: 404.
• After: Redirect to the canonical URI.

Maniphest Tasks: T13353

Differential Revision: https://secure.phabricator.com/D20688