Add severity meter to risk assessment MR widget

What does this MR do and why?

Adds a segmented severity meter to the risk assessment merge request widget so the risk level reads at a glance in the widget's title bar.

References

Thought (non-blocking): The completed state of the risk assessment doesn't give much direction to the user. They must determine what these badges mean in conjunction with the rationale.

#629032

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Open a merge request that has a risk assessment in an EE environment.
2. View the merge request widget for the risk classification section.
3. Confirm the severity meter renders in the title bar in place of the previous status icon, filling segments up to the current risk level, and pulsing while the assessment is still loading.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.