Stage 08 extensions and realms/logout

[danlester]

Fixes #1065

Changes:

• Renamed stage 08 to 08-qhub-tf-extensions
• Restored and simplified tf_extensions functionality
• jupyterhub.overrides.hub.extraEnvs is merged with native qhub env vars (restored functionality)
• Keycloak realm is called qhub again, not qhub-<project name> to simplify links
• Logout functionality restored - i.e. JupyterHub logout also logs out of Keycloak (and other tf_extensions if present)

Note that existing recent dev deployments may struggle with the realm renaming, and of course will need stages to be cleared before re-rendering to avoid bugs like #1062

Types of changes

What types of changes does your code introduce?

Put an x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features to not work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

Requires testing

• Yes
• No

In case you checked yes, did you write tests?

• Yes
• No

[costrouc]

Looks great to me. I was able to deploy with this branch successfully. Thanks for the changes @danlester

[costrouc]

I was wondering how to add this behaviour to conda-store. Awesome didn't think that logout could support a redirect allowing the user to do multiple logouts. Thanks :)