Add initial create screen for auth providers

Summary: Ref T1536. Adds an initial "choose a provider type" screen for adding a new provider. This doesn't go anywhere yet.

Test Plan: {F46316}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, chad

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6199