Improve lite gist handling

[bollwyvl]

References

• backport from try lite vs 4.1.0 etc #21 (but without the jupyak_config.toml)

Changes

• refactor lite dep/target finding
• if it exists, copy the whole gist into the lite_dir

  • allows for e.g. overrides.json

• merge more keys (e.g. federated_extensions)
• don't set ignore_sys_prefix
• add github pull request and issue templates

[github-actions]

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