Add Twitter as an authentication provider

Summary: Ref T3687. Depends on D6864. Implements the OAuth1 provider in Phabricator (which is mostly similar to the OAuth2 provider, but doesn't share quite enough code to actually extend a common base class, I think) and Twitter as a concrete subclass.

Test Plan:
Created a Twitter provider. Registered, logged in, linked, refreshed account link.

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Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3687

Differential Revision: https://secure.phabricator.com/D6865