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.
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