The `budgets📧selected` and `budgets📧unselected` tasks are
supposed to be run manually because they only make sense at a specific
point during the life of a budget.
However, they would only run on the default tenant, and it was
impossible to run them on a different tenant.
So we're introducing an argument in the rake task accepting the name of
the tenant whose users we want to send emails to.
fe9463cb5f