new vellum
Revert "new vellum"

This reverts commit d329bfe9e21a56993375d0e4c5ac8042fe58a455.
initial models for Accounting (BillingAccount, BillingContact, and Currency)
initial structure for testing the creation of an invoice, our goal
condense models in accounting
yes, please protect this relationship in billing account. don't delete the billing account. bad. bad.
move enums and choices into models
get_choices() -> CHOICES, and get rid of useless class in accounting
get rid of billing contact, just point directly to web user
added some indexing to certain fields
whitespace, where you come from?
software product, subscription, and rate models added to accounting
typo in help text for billing account’s salseforce id
generators useful for tests in accounting
test models for now, wait on testing invoice generation until the invoice model is set
add software product type and index on product type and feature type
added smsbillables app for billable refactor to corehq
get_or_create on currency_usd in accounting
instantiate software plans from our current list of plans in accounting, for use in tests and bootstrapping
new management command to boostrap accounting, starting with software plans