drag and drop UI for checksum search

Asking users to insert by hand the checksums of the files they want to lookup in SWH is annoying. We should implement a nicer UI that allows to drag & drop files on to our webapp and look them up. This will also make it easier to search multiple files at a time.

A sample UI that we might want to emulate / take inspiration from is that of Proof of Existence.

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Migrated from T357 (view on Phabricator)