Add the ability to search by request email address to the admin interface

[HelenWDTK]

Replaces #2692, deleted in error

On 30 July 2015 @crowbot wrote: At the moment there is only substring search on the request title. Would be useful for admins in redelivering the responses in the holding pen - often the request address is quite clear to a human reader but malformed in a way that prevents automatic delivery. Admins may not necessarily know how to identify the request from the address.

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On 3 November 2023 @garethrees wrote: We don't store the full address as a string in the database, so we'll probably have to cut it up to its components and do a search like where(id: id_extracted_from_full_address, idhash: idhash_extracted_from_full_address).