Fixes#15959
Introduced in #6540, OAUTH_REDIRECT_AT_SIGN_IN allowed skipping the log-in form
to instead redirect to the external OmniAuth login provider.
However, it did not prevent the log-in form on /about introduced by #10232 from
appearing, and completely broke with the introduction of #15228.
As I restoring that previous log-in flow without introducing a security
vulnerability may require extensive care and knowledge of how OmniAuth works,
this commit removes support for OAUTH_REDIRECT_AT_SIGN_IN instead for the time
being.
When using a CAS server, the users only have a temporary email
`change@me-foo-cas.com` which can't be changed but by an
administrator.
We need a new environment variable like for SAML to assume the email
from CAS is verified.
* config/initializers/omniauth.rb: define CAS option for assuming
email are always verified.
* .env.nanobox: add new variable as an example.
Claire