Exclude redundant fields in Roller
Exclude redundant fields in Roller

Use the ``exclude()`` method on the querysets and lists built with
a ``build_exclude_fields()`` method to get rid of redundant fields.

Collect and reference the querysets used by the Roller in a somewhat
more declarative, DRY way.

Also, sort the fields alphabetically.

Addresses #39

datamade/openelections-core,datamade/openelections-core,cathydeng/openelections-core,openelections/openelections-core,cathydeng/openelections-core,openelections/openelections-core