Expand libphutil support for Twitter as an auth provider

Summary:
Ref T3687. Companion to the next diff, which adds the Phabricator half of this. Twitter is an easier OAuth1 proivder to implement and get a sanity check from, on the road to JIRA. Broadly:

  - Consumer secrets are now wrapped in a `PhutilOpaqueEnvelope` for consistency with OAuth2.
  - Twitter provider now returns full account info (username, profile image, etc), by making a call to the Twitter API.

Test Plan:
Registered using Twitter, got a registration form filled with information:

{F57054}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3687

Differential Revision: https://secure.phabricator.com/D6864