Update Authentication to use OAuth 2.0

[noeltock]

Trying to authenticate new accounts results in an error linking you to:
https://developers.google.com/identity/protocols/AuthForInstalledApps

This in turn informs that ClientLogin is dead and that we have to use:
https://developers.google.com/identity/protocols/OAuth2

Till this is fixed, no one case authenticate a new account on Top Posts.

Edit:

Robert O'Rourke [18:23]
there’s code in the TJX repo that uses the latest google API php lib which could be pulled out and refactored a bit to fit more or less

[mattheu]

Worth looking at the 'refactor' branch - I did a bunch of work including using a newer version of the Google PHP API. It was a while ago though!