new DefaultProductPlan model for accounting to map a default plan to a product_type
instantiate community plans and differentiate from subsscribable plans in accounting generator
new InvoiceError exception in accounting
get product type for a domain
some billing accounts might be generated on the creation of an invoice
blank=True should also have null=True in accounting model fields
increase max length of product_type
two different invoice factories, one to create invoices on existing subsciptions, and one to see if an invoice should be created for unsubscribed domains based on whether they have a balance for that month—tests are still in the works
line items associated with that zero balance invoice need to be deleted before the invoice is.
make sure to instantiate subscribable plans and community plans in tests