User edit page times out with large numbers of users & trusted contacts enabled

Following discussion in #2151631, it turns out that the user_trusted_contacts field is actually an options select list, thus the need to query on all referencable entities. This isn't apparent without looking at the field definition because this is meant to be used programmatically and we hide it in the UI.

So, the change here is to make the widget type for user_trusted_contacts an autocomplete widget, even though users should never see the field :). Adding to the Commons 3.6 radar and marking as "critical" since it's pretty disruptive to be unable to edit user accounts.