R1:1767b80654fe

Replace manual query string construction with "phutil_build_http_querystring()"

Summary: Now that we have a nice function for this, use it to simplify some code.

Test Plan: Ran through the Duo enroll workflow to make sure signing still works.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20053