🔀 Implement issue conversion rate tracker

[janhalen]

🙍🏻‍♂️ Story:

As a project stakeholder,
I want to measure and visualize the number of issues that ends up with actual changes to the product.
So that I can assess the overall engangement and idea conversion ability of the projects community.

✅ Acceptance criteria:

1. A metric that shows the number of issues that have linked pull requests (PRs) merged to the main branch is available.
2. A graphical representation the total number of issues and the number of issues with linked, merged PRs canbe shown.
3. The representation shows the conversion rate of issues to merged PRs as a percentage.
4. One of 3 categories are applied to the metric from a set of predefined ranges and is illustrated with a red/yellow/green colour depending on the level.