Automation of end-date based on start-date

Hey,
@nursix Please Review
@flavour Removed the seperate js file and put in s3.datepicker.js
If you will, the actual, primary meaning of the ToDo is to relieve the user of the end date calculation - and to work with the parameters he's typically given.

In my eyes, entering a concrete end-date is an edge-case - yet the form demands it, with or without automatic calculation from a site-wide standard interval. Thus it doesn't really support the user - and to the contrary, the standard process in the user's mind would be to verify the automatic value, hence he would still have to perform the calculation.

If, by contrast, the user can enter the interval instead of the end date *then* you've really made a difference here.