Add header to Slack report which includes other details

[adamrtalbot]

Some details are shared by all pipeline runs, such as environment, deployment, version number etc. We should add these to the Slack hook so we can see them on all Slack posts.

[adamrtalbot]

This is blocked because we have run out of space to nicely render the output.

We need to come up with a better solution for sending to the Slack API.