OAuth - Phabricator OAuth server and Phabricator client for new Phabricator OAuth Server

Summary:
adds a Phabricator OAuth server, which has three big commands:

• auth - allows $user to authorize a given client or application. if $user has already authorized, it hands an authoization code back to $redirect_uri
• token - given a valid authorization code, this command returns an authorization token
• whoami - Conduit.whoami, all nice and purdy relative to the oauth server.

Also has a "test" handler, which I used to create some test data. T850 will
delete this as it adds the ability to create this data in the Phabricator
product.

This diff also adds the corresponding client in Phabricator for the Phabricator
OAuth Server. (Note that clients are known as "providers" in the Phabricator
codebase but client makes more sense relative to the server nomenclature)

Also, related to make this work well

• clean up the diagnostics page by variabilizing the provider-specific

information and extending the provider classes as appropriate.

• augment Conduit.whoami for more full-featured OAuth support, at least where

the Phabricator client is concerned

What's missing here... See T844, T848, T849, T850, and T852.

Test Plan:

• created a dummy client via the test handler. setup development.conf to have

have proper variables for this dummy client. went through authorization and
de-authorization flows

• viewed the diagnostics page for all known oauth providers and saw

provider-specific debugging information

Reviewers: epriestley

CC: aran, epriestley

Maniphest Tasks: T44, T797

Differential Revision: https://secure.phabricator.com/D1595