Make the calendars controller a little easier to understand

This shows some points for future extractions
Calendars Controller: Make the borders cleaner
The day presenter should take a date and the info if it has events

Not a hash
SingleEvent#events_per_day_in(date_range)

Return the number of single events per day in a certain date range
This is untested due to #318
Real implementation of the generated day list

No tests... But the entire controller has no tests.
This will be my first controller to tackle.
I think this will be easier now after the reorganization.