Allow per project configuration for disabling Slack thread grouping

### Problem Statement

Right now, Slack thread grouping can only be toggled org wide. Some teams prefer individual alerts in channel (no threads) to avoid losing visibility of non Sentry conversations

### Solution Brainstorm

Allow for specific Sentry projects to disable slack thread grouping

### Product Area

Settings - Integrations

The issue identifies Settings - Integrations and the Slack thread-grouping behavior, but names no files or tests. Start by locating the existing organization-wide Slack thread-grouping setting and its alert delivery path. Done means individual Sentry projects can disable grouping without changing the organization-wide behavior for other projects.

Written by the indexing model from the issue text.