Develop and test script to list usernames that will get escaped differently with KubeSpawner 7's safe slug scheme

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I think it can be useful for the upgrade guide to include a script to check username expansion differences. This could be useful in 2i2c deployments for example, so I figure I'll develop it anyhow.

• z2jh changelog/migration notes

  • see a specific migration step in the end of add 'safe' slug scheme #744 (comment)
  • script to look at usernames etc considered in add 'safe' slug scheme #744 (comment) considered