Add sharding keys to slack_integrations table

What does this MR do and why?

This MR creates columns to be used as sharding keys for the slack_integrations table.

References

#524680

How to set up and validate locally

Setting up Slack

https://docs.gitlab.com/user/project/integrations/gitlab_slack_application/

• Created a free Slack workspace.
• Within that workspace, create an app: https://api.slack.com/apps/
• Enter the app details in your GDK admin panel: http://gdk.test:3000/admin/application_settings/general
• Under http://gdk.test:3000/admin/application_settings/integrations, enable the GitLab for Slack app
• Click "Allow" when prompted on Slack. You should be successfully redirected back to your GDK.

Checking ID propagation

• After installing the Slack app at the instance level, you should have an instance level integration, and one for each project and group.

Group.count + Project.count + 1 == SlackIntegration.count
# => true

• The sharding key of each integration SHOULD correspond to the organization ID (for the instance level integration) or the project/group to which it belongs.

SlackIntegration.pluck(:id, :organization_id, :group_id, :project_id)

correct_values = SlackIntegration.all.all? do |integration|
  expected_alias = if integration.organization_id
    "_gitlab-instance"
  elsif integration.group_id
    Group.find(integration.group_id).full_path
  elsif integration.project_id
    Project.find(integration.project_id).full_path
  end
  
  integration.alias == expected_alias
end

if correct_values
  puts "✅ All sharding keys correspond to the expected alias"
else
  puts "💣 Mismatch between sharding key and alias"
end

• Manually create a group and a project. The last two SlackIntegration records should have the correct sharding key set, verifiable with the above script.

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