R1:be66a52050fd

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

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