Convert email contact "to" boxes to Entity Ref fields
Currently, the email "to" boxes in the email contact form use an older form of search. It would be a better user experience to convert this to using the Entity Ref search box from here: https://github.com/civicrm/civicrm-core/blob/db9e0947e008ba06308da5664e5c36a3bc2736df/js/Common.js#L739.
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