Upgrade to a newer Google OAuth API

[sffc]

The current sign-in API used by Rietveld uses an older sign-in screen, which has been off-putting to some members of our team who are concerned about their account security:

At the bottom, it says that the application only needs your email address, but the line about "permission to access your Google account" sounds a little scary. Newer sign-in APIs have a more inviting sign-in screen:

It would help us onboard more members of our team to Rietveld if the more modern sign-in screen were used.

[cedk]

This is managed by the appengine (and not the rietveld code).
By the way, on my instances I got the new login page.