Add chamber to Office model, allow 'US' for state
Add chamber to Office model, allow 'US' for state

We need this to allow importing records from CSV in the
``load_metadata.run`` task.

Serdar suggested using 'US' as the state key for national-only
offices such as President.

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