Preserve "next" URI by using OAuth 'state' parameter

Summary:
When a user clicks a link like /T32 and has to login, redirect them
to the resource once they've authenticated if possible. OAuth has a param
specifically for this, called 'state', so use it if possible. Facebook
supports it but Github does not.

Test Plan:
logged in with facebook after viewing /D20

Reviewed By: aran
Reviewers: aran
CC: aran, epriestley
Differential Revision: 61