Slack notifications for support escalations

As of this PR, the engineering team has a Slack channel with the support help desk that notifies us when there's an escalation that needs actioning. The below documents our Slack integration.

The escalation works by sending a POST request to a Slack webhook that then sends a message to the relevant support escalation channel. This is managed within a Slack app that the engineering team has joint membership over.

To get the webhook token for the existing channel (or generate a webhook for a new channel / testing), navigate to Add Features and Functionality > Incoming Webhooks. There, you can see the list of existing webhooks and generate new ones.

In our application.yaml, our Slack hook token is set to foo as a default. If you want to test the integration locally, replace the default string with the part of the webhook token after the base URL (ie the part of the URL that makes the Webhook unique). In Azure Key Vault, this value in prod / test is set as slack-hook-token-{env}.