Add feature flag for Vue merge request search dashboard

What does this MR do and why?

This merge request adds the mr_dashboard_vue_search feature flag and the controller, helper and view changes that mount the Vue dashboard on the merge request search page. With the flag off the legacy HAML search page renders exactly as before, so this merge request changes nothing for users.

The controller normally rejects a request with no filters. Behind the flag that check is skipped for the search action, so the client can apply its own guard.

References

Split from !252201 (closed)

Part of a stack:

1. !252305 (merged) - (this merge request) feature flag and backend enablement
2. !252306 (merged) - Vue search list
3. !252307 (merged) - remaining filter tokens
4. !252308 (merged) - row details
5. !252309 (merged) - approval status

Screenshots or screen recordings

Before	After

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.