Added a red notification thunderbolt for when we're on an admin page.

Summary: The blue thunderbolt looked out of place on the red admin pages; now it matches the colors from the admin Phabricator logo.

Test Plan: Go to an admin page, see a red thunderbolt. Go to a not-admin page, see a blue thunderbolt.

Reviewers: epriestley

CC: aran, Korvin

Maniphest Tasks: T1427

Differential Revision: https://secure.phabricator.com/D2895