Default notifications to team slack channels

### Problem Statement

When a new issue is assigned to a team, we need to send a slack notification to their feed.

The feed channel is a slack channel dedicated to a given team that is monitored by the team's duty. The team's duty is never the same user. The goal of the team's duty is manage the RUN. Therefore seing new Sentries is key for them.

As today with the Slack integration, individual user receive Slack alerts (with the general alert notifications settings).

Another possibility is to use alerts, however they need to be defined team by team and project by project. This can lead to a lot of configuration (50 teams, with 3 projects each thats 150 alerts to setup...)

### Solution Brainstorm

The Slack integration already allows connecting a team to a given Slack channel (not that it could also be applied other tools such as Teams)

Ideally we would need a way to setup notifications for a team (similar ui as in "Settings" > "Account" > "Notifications" but for a team).

### Product Area

Settings - Integrations