pi: Add RFP billing status validation.

[lukebp]

One thing that we missed in the original billing statuses PR was the fact that RFP parent proposals don't have a billing status since the do not request funding and are never billing against the treasury.

• Validation needs to be added to the backend that prevents billing status changes on RFP proposals.
• The frontend needs to not display the "Set Billing Status" button for RFP parent proposals.