Slack Integration for Notifications

### Description

Presently if FlowFuse raises a crash/restart/safe mode event from a Node-RED instance, the alerting consists of either email, or a notification in the FlowFuse API.

However, it is not always the case that email can be setup due to a company's security policy, and there's not always going to be someone monitoring the FlowFuse API 24/7 to check for alerts.

An alternative to this would be to include Slack integration in FlowFuse so that alerts could configured to be sent to a Slack channel.

Example of setting up an incoming Webhook in Slack can be found here: https://api.slack.com/messaging/webhooks

### Which customers would this be available to

Everyone - CE/Starter/Team/Enterprise

### Have you provided an initial effort estimate for this issue?

I am not a FlowFuse team member