Use creation date for displaying issues without start date on the calendar
We removed the Standard Fields "Start Date" and "Due Date" from one of our Trackers.
Now new Issues don't show up in the Calendar.
Looking at the code I discovered that redmine uses the start_date field to search
for Issues in the Database.
I think it would be better to use created_on if start_date is nil or fill start_date
with current date if it is nil in the controller on creation.
This way we could keep using the great Calendar, but also can remove unneeded fields from