Fix transaction title rendering for AuthenticationConfigs

Summary: I was poking around in PhabricatorAuthProviderViewController and noticed that none of the subclass-specific rendering was working. Figured out that no one ever calls PhabricatorAuthProviderConfigTransaction->setProvider(), so instead of adding all those calls, just pull the provider out of the config object.

Test Plan:
Before:

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D20655