Support notifications and two-way Slack threads

Support activity now sends email: your team is notified when a visitor opens or replies to a thread, and the visitor is notified when your team answers. Each project can also connect a Slack workspace and channel from Settings, which mirrors the whole conversation into a Slack thread (including AI answers, closes, and reopens) and sends replies written in that thread back to the customer. Every notification email includes a link to turn off notification emails for your own account.