make Fieldmapper provide hints regarding cardinality of links

Issues like bug 1239822 and bug 1269042 suggest that it would be useful to centralize the decision of whether to use a text, auto-completing text, or dropdown widget for specifying records from a linked table. Having a central place (i.e., fm_IDL.xml) would avoid having to scatter one-shot code that (e.g.) prevents doing things like constructing a drop-down of all copy IDs in the system.

An idea from Mike Rylander is this: define a "oils_persist:cardinality" attribute for links with values of "low", "high" or "unbounded", which could be used to set the type of widget (dropdown for low, autocomplete for high, nothing for unbounded).

Evergreen master