Refactor vulnerability header and footer

What does this MR do?

This MR refactors the vulnerability header and footer to prepare it for the history list feature. It does the following:

1. Renamed app.vue to header.vue to match with footer.vue and updated its component name from VulnerabilityManagementApp to VulnerabilityHeader to match with VulnerabilityFooter.

2. Fixed some spacing issues and removed some unnecessary wrapper elements.

3. Moved the issue note from below the horizontal line to above it. This is to avoid confusion because the history list will be below the line, so it makes more sense to have the "either there's only one item shown or not at all" stuff to above the line:

Before	After

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability 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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done