Clarify crossed out vulnerabilities in security MR widget

This is an actionable insight from the UX Department: MRs experience async critique research (insight).

What

In the security MR widget, sometimes vulnerabilities are crossed out without any explanation.

How

1. Remove crossed out treatment of Dismissed vulnerabilities

2. Add the Dismissed badge (as shown in the vulnerability detail modal)

Implementation Plan

• Modify app/assets/javascripts/reports/components/report_item.vue, remove the is-dismissed class, and change it to a GlLabel or GlBadge instead (whichever component makes more sense).
• Add tests.
• Update the docs (screenshots, wording, etc).