Fix a few minor bugs in Auth Providers

Summary: Fixes T7496, T7511. Sets text for registration is not enabled, sets can_manage on add_provider button.

Test Plan: Test with a logged in admin and logged in normal joe user.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7496, T7511

Differential Revision: https://secure.phabricator.com/D12014