search: preserve scroll position on navigating back to search results 

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Created by: valerybugakov
The scroll position is not preserved when going back to search results.
TLDR The scroll position is now preserved for page refresh and back navigation
This PR does two things:
ScrollManager
that tracks the scroll position of the search results container, persists it to session storage, and restores it on back navigation or page refresh.
itemsToShow
state used in StreamingSearchResultsList
.
use-items-to-show.test.ts
with tests for all important use-cases.https://user-images.githubusercontent.com/3846380/146541677-b48456d9-eaa5-42bd-a550-6fbeb97fc248.mov
Should be merged together with https://github.com/sourcegraph/sourcegraph/pull/29176. Closes https://github.com/sourcegraph/sourcegraph/issues/28845.
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