Add fallback to message cost with null account and tag pool name in transaction creation.
Merge branch 'feature/issue-885-clean-up-billing-api-tests' into feature/issue-881-default-message-cost
Merge branch 'develop' into feature/issue-881-default-message-cost
Allow tag_pool to be optional so that a global default message cost can be set.
Enable falling back to global account and tagpool independent message cost.
Add help_text for account, tag_pool and message_direction on the MessageCost model.
Avoid line-continuation slash. Brackets ftw.
Add indexs and uniqueness constraints to message cost table.
Add check for message_direction on fallback message cost path.
Check that if a message costs account is not None, neither is its tag pool.
Add test that message costs with account numbers but no tag pool cannot be created via the billing API.
Add indexes and constraints to the MessageCost model and allow tag pool to be null. This migration is irreversible (since we cannot do anything sensible with null tag pools).
Merge branch 'feature/issue-881-default-message-cost' into develop