Before Budget.current could return multiple budgets, now there can only
be a single current_budget.
Adding the concept of open, which better reflects what the admin sees
in this page: A tab for open budgets and a tab for finished budgets
Why:
* As seen on preproduction and production environments on Madrid's fork. Budget::Result#calculate_winners is very costly when done to all headings for a given budget (as requested on Admin::BudgetsController#calculate_winners) but its not when done individually for only a heading (as requested on Budget::Investment#recalculate_heading_winners)
How:
* Removing `handle_asynchronously :calculate_winners` from bellow Budget::Result#calculate_winners definition, to avoid making any call delayed. And explicitly calling `.delay` only when needed (on Admin::BudgetsController#calculate_winners)
