Move setting of primary party out of _build_meta_fields()
Move setting of primary party out of _build_meta_fields()

The way that the row is fetched will vary from file to file, making
it hard to move this method to BaseLoader if we want to do that
in the end.

Addresses #49

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