New diffusion repository rejection should not navigate away from diffusion.

Summary: Fixes T7507, Create diffusion repo rejection should not navigate away from diffusion.

Test Plan: Login as non-admin, open diffusion, attempt to create new repo, rejection dialog should appear over page instead of navigating to new page.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7507

Differential Revision: https://secure.phabricator.com/D12557