Fix broken suggestion/validation for Owners paths in repositories with short names

Summary:
Depends on D19189. Ref T12590. The "validate" and "complete" endpoints for this UI could incorrectly return redirect responses. These aren't critical to the behavior of Owners, but they're nice to have, and shouldn't redirect.

Instead, skip the canonicalizing redirect for AJAX requests.

Test Plan: Edited Owners paths in a repository with a short name, got completion/validation again.

Maniphest Tasks: T12590

Differential Revision: https://secure.phabricator.com/D19190