SearchController rename search_service instance var

What does this MR do and why?

Related to #378012 (closed)

Rename the @search_service instance variable in the SearchController to @search_service_presenter - that is what it actually contains.

Update usage in views and update specs

Screenshots or screen recordings

N/A

How to set up and validate locally

1. make sure gdk is setup for elasticsearch and all data in indexed
2. verify search without a search term: http://gdk.test:3000/search
3. verify search with a search term for Advanced Search: http://gdk.test:3000/search?scope=blobs&search=test&project_id=6
4. verify search with a search term for Basic Search: http://gdk.test:3000/search?scope=blobs&search=test&project_id=6&basic_search=true

MR acceptance checklist

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