Keycloak Export

[danlester]

Fixes #919

Changes:

• Added a scripts/keycloak-export.py script to export users/groups from Keycloak
• Documentation added to 'Manual Backups'
• Also talk about JupyterHub DB backup in the docs

Types of changes

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• 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
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Testing

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[danlester]

BTW the 'dead link' detected by Vale won't be dead when this PR is merged!

[tylerpotts]

LGTM