Add missing "Filters" toggle button to Zoekt code search results

What does this MR do and why?

Adds the missing "Filters" toggle button to Zoekt code search results page.

References

Fix missing mobile "Filters" toggle button on e... (#575836 - closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Perform a code search in a project/group that is indexed by Zoekt.
• On a big screen, see the filters as sidebar on the left.
• On small screens, find the fixed "Filters" button on the top-right.
• Clicking it should show the filters. Clicking it again hides the filters.

MR acceptance checklist

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