Add a breakdown by severity to the summary of Slack team reports

[tarkatronic]

🚀 Feature Request

Is your feature request related to a problem? Please describe.

It's currently difficult to see, at a high level, what the breakdown of a team's findings are. We have a summary line, but it only provides an overall number.

Describe the solution you'd like

Alongside the total number of vulnerabilities, we should have another text block breaking them down by severity.

Additional context

This is a follow-up to #56, as proposed in #46.