Security dashboard should show dismissal details on issues

What does this MR do?

Adds a new box that shows dismissed details on a vulnerability to the vulnerability modal.

This joins the issue and merge request creation information if it exists.

The information displayed in this box is slightly different depending on the information that's available to it at the time it's rendered. For example, if the vulnerability was dismissed on the Group Security Dashboard, we'll have project information, but not pipeline information. If it was dismissed on the Merge request or pipelines pages, the opposite is true.

A follow up issue/MR will add timestamps to these info boxes.

I've also refactored the <event-item> component to make it a bit more generic and easier to re-use. This is in preparation for gitlab-ui#229 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Closes #9715 (closed)