Fix bug in Extensible.form.recurrence.option.Weekly

Extensible.form.recurrence.option.Weekly did not properly initialize the seven checkboxes in some case.

To reproduce bug: Create a weekly event A without selecting any checkboxes. Create a weekly event B with selecting multiple checkboxes. Open event B in details view. Then close and open event A in details view. The checkboxes will be set wrong. 

To fix, initialize checkboxes each time when widget displayed.