Merge users (migration to Enterprise Grid changes account IDs)

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A slack user reported appearing twice in a leaderboard. Turned out this was caused by a migration to enterprise grid, their ID has changed.

https://slack.com/intl/en-au/help/articles/115002532808-Migrate-workspaces-to-Enterprise-Grid
https://api.slack.com/enterprise/grid/developing#migration

The easiest way to fix this is probably to merge users, in the direction of the new account.

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Another problem is when the channel is added to a different slack workspace, the bot still responds in the channel but the team ID has changed. If someone reinstalls the bot, they now have 2 bots 😠