Fix missing _ import.

Fallout from #1792.
Fix another unbound underscore
Merge pull request #1925 from edx/christina/underscore

Fix missing _ import.
Merge pull request #1862 from carsongee/bugfix/mitx/external_auth-retfun

Fix external_auth to properly use retfun for @ssl_login_shortcut()
Add additional check signin to make sure an SSL certificate is passed
before redirecting to SSL authentication.