This replaces the two custom Google authentication backends originally
written in 2012 with using the shared python-social-auth codebase that
we already use for the GitHub authentication backend. These are:
* GoogleMobileOauth2Backend, the ancient code path for mobile
authentication last used by the EOL original Zulip Android app.
* The `finish_google_oauth2` code path in zerver/views/auth.py, which
was the webapp (and modern mobile app) Google authentication code
path.
This change doesn't fix any known bugs; its main benefit is that we
get to remove hundreds of lines of security-sensitive semi-duplicated
code, replacing it with a widely trusted, high quality third-party
library.
In auth setup, if you're at the error page you may be inclined to
refresh the page once setting up auth after restarting the server,
but the error page is a static page, so add instructions to go
back to the login pages again and try the auth flow from there.
Harshit Bansal
Vishnu Ks
Tim Abbott
Brock Whittaker
Vaida Plankyte