Mark external -> external redirects in Phame to canonicalize URIs as "external"

Summary: Ref T9897. If you visit /post/123/spoderman/ it will try to redirect you to /post/123/spiderman/, but currently only internal views work because these redirects aren't marked as safe/external.

Test Plan: Visited a misspelled/out-of-date URI on an external blog view, got a good redirect.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9897

Differential Revision: https://secure.phabricator.com/D14741