Add search and filter capabilities to dashboard issues refactor

What does this MR do and why?

This MR adds the ability to search and to filter by assignee and author for the dashboard issues refactor

No changelog required as it is behind feature flags vue_issues_dashboard and root_level_issues_query, default disabled

Follow-up MRs will add more filter tokens etc.

Screenshots or screen recordings

Screen_Recording_2022-12-07_at_6.21.40_pm

How to set up and validate locally

1. In rails console enable the feature flags

   Feature.enable(:vue_issues_dashboard)
   Feature.enable(:root_level_issues_query)

2. Visit the issues dashboard page http://127.0.0.1:3000/dashboard/issues
3. Test searching and filtering in the filter bar

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.