Slack is the app you've got open all day at work, and there's a good chance nobody at your company is actually in charge of buying it. That little detail ended up steering one of the biggest software deals ever, Salesforce's $27.7 billion purchase of Slack. Here's how most work software gets bought. Your head of sales picks the CRM and pays for it out of the sales budget. Your CMO does the same thing with the marketing tools. There's one person to convince, and the vendor knows exactly who to…