Make conduit read access_token and login the pertinent $user

Summary: This makes the oauth server a bunch more useful.

Test Plan:

• used /oauth/phabricator/diagnose/ and it actually passed!
• played around with conduit via hacking URL to include access_token on a logged

out browser

• linked my account to itself by going to /settings/page/phabricator/, clicking

"link" account, then cutting and pasting the pertinent ?code=X into
/oauth/phabricator/login/.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Maniphest Tasks: T852

Differential Revision: https://secure.phabricator.com/D1644