feat!: review external commit [CL-90]

[beltram]

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PR Submission Checklist for internal contributors

• The PR Title

  • conforms to the style of semantic commits messages¹ supported in Wire's Github Workflow²
  • contains a reference JIRA issue number like SQPIT-764
  • answers the question: If merged, this PR will: ... ³

• The PR Description

  • is free of optional paragraphs and you have filled the relevant parts to the best of your ability

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What's new in this PR?

Issues

• join_by_external_commit returns a valid PublicGroupState which becomes valid when the external commit gets merged
• Group configuration is carried in PublicGroupState so that it becomes freestanding and one has not to fetch a configuration from its custom Delivery Service

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References

1. https://sparkbox.com/foundry/semantic_commit_messages
2. https://github.com/wireapp/.github#usage
3. E.g. feat(conversation-list): Sort conversations by most emojis in the title #SQPIT-764.