Add the option to select a subset of fields in a category for canonicalization

[offbyone]

The example here is something like Best Fan Writer, where only the writer's name field should be considered for canonicalization (ie, field_1) but we want to include field_2 on the ballot for informational content.

• add a property to the nominate.Category model that identifies the fields that are used for canonicalization
• modify the method that proposes a work name to refer only to the specified fields