Admin search features aren't comprehensive #630
Description
Some of the search methods in the admin controllers aren't checking all the fields that a user might want to use to search for that kind of content.
Also, in some cases it would make sense to search against fields from associated objects, which none of the searches currently support - and some of those relationships are polymorphic. For example you might well want to search on /admin/lists/1/subscriptions for a ShinyList::Subscription by putting in the name or email address of the subscriber, which will be either a User or an EmailRecipient.