Test Sourcegraph with Atlassian Identity Manager

Created by: beyang

Test Sourcegraph with Atlassian Identity Manager SAML: https://www.atlassian.com/blog/announcements/introducing-identity-manager-atlassian-cloud.

$CUSTOMER mentioned some difficulty in setting up Sourcegraph with Atlassian Identity Manager with SAML authentication. We should test this out and add to the SAML docs (and potentially improve docs around general SAML debugging, as well).