Frontend for multiple matches - part-1

What does this MR do and why?

This MR is a part of bigger issue Frontend for multiple matches in single file.

This establishes the basics feature flag and the changes in the Haml to initiate the Vue component.

MR	status
Frontend for multiple matches - part-1	👈 you are here
Frontend for multiple matches - part-2	merged
Frontend for multiple matches - part-3	merged
Frontend for multiple matches - part-4	in review
Frontend for multiple matches - part-5	ready to merge

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enable the forks filter FF

   echo 'Feature.enable(:zoekt_multimatch_frontend)' | rails c

2. Make sure advanced search is installed and enabled
3. Make sure you enable zoekt.
4. Visit search page http://gdk.test:3000/search
5. Search code using namespace where zoekt is enabled
6. At this point you should see empty page

Related to #450735 (closed)