misc/origin_save: Improve save requests page retrieval performance
Improve the overall performance when one wants to display a specific page of the save code now requests list by:
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Applying possibly costly operations (like finding the visit date) only on the save requests to display (the whole list was considered previously)
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Implementing search feature using Django model instead of pure Python
Also add missing tests.
Migrated from D4816 (view on Phabricator)