Straighten out absolute/relative URIs in login providers

Summary:
Ref T4339. Login providers use absolute URIs, but the ones that rely on local form submits should not, because we want to include CSRF tokens where applicable.

Instead, make the default be relative URIs and turn them into absolute ones for the callback proivders.

Test Plan: Clicked, like, every login button.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4339

Differential Revision: https://secure.phabricator.com/D8045