Update Confluence 2fa login page for New Enrollment

Summary:
- fixed size iframe that will support both new enrollment and classic (requires a css change to new enrollment that ashalabi already made)

- removes the border from the iframe

- centers the iframe horizontally

Classic looks pretty bad without the frame border.  I will add that back in with the confluence.css in TP.

Test Plan:
Run local confluence with plugin.

Will add some screenshots.

Reviewers: karl, omar, ashalabi

Reviewed By: ashalabi

Maniphest Tasks: T3466

Differential Revision: http://phab.duo/D1483