new member QR code

[sharrisoncaktus]

• adds a QR code at the point in the create member flow where the new member needs to go to a new URL to authorize the org and create a password.
• nicety: better labels on that final form for the passwords.
• fix the styling of the modal: "min-height: 100%" rather than "height: 100%" fixed the background overflow issue that I was seeing
• new utility function to make QR codes: apps.org.utils.make_qr_code(text)
• unit test for that function, using a known good base64 encoded PNG
• SKIP the additional_info form for now since it's empty; easy to wire it back in later