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.
Previously, we used to raise an exception if the direct dev login code
path was attempted when:
* we were running under production environment.
* dev. login was not enabled.
Now we redirect to an error page and give an explanatory message to the
user.
Fixes#8249.
Harshit Bansal
Vishnu Ks
Umair Khan
Tim Abbott
Steve Howell
Vaida Plankyte