FR: Custom metadata Enum of all users

[madduck]

With reference to #1029, here is a use-case for an enum field type: users. If I need a field that tracks e.g. which user currently has the document, or who is currently responsible for handling it, then it would be good to be able to select only among the available users, rather than having to type in their names (which means spelling errors)…

[madduck]

With such a field type, overloading the context-specific operator : would enable f:assigned:me to show all documents that had me in the custom field "assigned".