Address code complexity, and Rubocop for global_slack_handler.rb

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The following discussions from !118289 (merged) should be addressed:

• @harsimarsandhu started a discussion: (+1 comment)

  How about we move the below code to a separate method to reduce cognitive complexity? something like

      def handle_command(slack_integration)
        chat_user = ChatNames::FindUserChatService.new(params[:team_id], params[:user_id]).execute
        integration = slack_integration.integration
  
        if chat_user&.user
          Gitlab::SlashCommands::Command.new(integration.project, chat_user, params).execute
        else
          url = ChatNames::AuthorizeUserChatService.new(params).execute
          Gitlab::SlashCommands::Presenters::Access.new(url).authorize
        end
      end

• @tkuah started a discussion:

  Follow-up: Should we move this into the model to address this Rubocop ?

/cc @.luke