Add action dropdown to bulk vulnerability updates

What does this MR do and why?

Related #408361 (closed)

This adds an action dropdown in the selection summary component behind a new feature flag enhanced_vulnerability_bulk_updates.

It has 1 option "Change status", and upon clicking that option, the status update form for vulnerabilities is shown.

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Before	After

How to set up and validate locally

1. Enable feature flag: http://gdk.test:3000/rails/features/enhanced_vulnerability_bulk_updates
2. Import security reports
3. Run pipeline on master
4. Go to vulnerability report and select some vulnerabilities
5. Validate there is a "Select action" dropdown. Validate it has 1 option "Change status".
6. Select that option and validate it now shows the "Change status" dropdown.