Scope down slack_integrations unique index

What does this MR do and why?

⚠️ #524680 needs to be completed first, before this MR can be merged, as we need all rows to have a sharding key set.

This MR removes existing index_slack_integrations_on_team_id_and_alias as requested in #562083 as it needs to be scoped down in order to support Cells Architecture. It replaces removed index with 3 new unique indexes, scoped by the sharding key:

• index_slack_integrations_on_team_id_alias_project_id
• index_slack_integrations_on_team_id_alias_group_id
• index_slack_integrations_on_team_id_alias_organization_id

References

#562083

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