Improve consistency of policy enforcement on new repository edit UI

Summary: Ref T2231. The policy rules are a little murky right now: the "Edit Repository" link requires CAN_EDIT, but the actualy page doesn't. Instead, require CAN_EDIT for the edit page.

Test Plan: As a user without CAN_EDIT, viewed a repository and clicked the edit link.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2231

Differential Revision: https://secure.phabricator.com/D7406