Correct Phorge icon on Auth Provider page

Summary:
Add Phorge icon to auth/login PNG files.

Closes T15403

Test Plan: Go to /auth/config/new/ in browser and look at icons.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: avivey, speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15403

Differential Revision: https://we.phorge.it/D25255