Made burndown chart a bit more reusable
Burndown chart visual improvements

* Add month markers
* Don't mark weekends if more than 3 months timespan is shown
Minor burndown chart fixes

* Wrap items/effort radio buttons so several charts work on same page
* Fix 'uninitialized value in multiplication' warning
Use max value as start point for ideal burndown line

This makes it work better with remaining/actual graph as items often
don't have the effort estimates from the beginning