Fix: calendar feed parameter and validation
Fix: calendar feed parameter and validation

The name of the parameter passed to
Concrete\Controller\CalendarFeed::view is `calendar_id`, so `identifier`
is not being passed. Additionally, passing a non-numeric value throws
an exceptions, so adding `setRequirements` back will validate the
input. It was taken out when this was refactored in 20eb23ca339.

MrKarlDilkington/concrete5,deek87/concrete5,deek87/concrete5,mlocati/concrete5,haeflimi/concrete5,biplobice/concrete5,olsgreen/concrete5,jaromirdalecky/concrete5,MrKarlDilkington/concrete5,haeflimi/concrete5,biplobice/concrete5,rikzuiderlicht/concrete5,hissy/concrete5,mlocati/concrete5,concrete5/concrete5,jaromirdalecky/concrete5,concrete5/concrete5,hissy/concrete5,mainio/concrete5,jaromirdalecky/concrete5,MrKarlDilkington/concrete5,rikzuiderlicht/concrete5,mlocati/concrete5,concrete5/concrete5,hissy/concrete5,hissy/concrete5,olsgreen/concrete5,rikzuiderlicht/concrete5,olsgreen/concrete5,concrete5/concrete5,mlocati/concrete5,mainio/concrete5,biplobice/concrete5,jaromirdalecky/concrete5,haeflimi/concrete5,deek87/concrete5,biplobice/concrete5,mainio/concrete5,haeflimi/concrete5,deek87/concrete5