Create a new custom rate for an organization
Creates a custom rate for the organization.
If no `start_date` and `end_date` are given, the start of the current month will be used as a `start_date` and will have no `end_date` and will therefore be active indefinitely.
If a custom rate for the same organization and ratezone exists and overlaps with the validity period of the new rate, the existing custom rate will have its `end_date` set to the last day of the month before the `start_date` of the new rate.