Automation of end-date based on start-date

Hey,
@nursix Please Review
@flavour Removed the seperate js file and put in s3.datepicker.js
That again is a common case - there are multiple forms (e.g. projects, activities, deployments, missions...) where we ask the user for start and end date while he more often has a start date and a time interval, so this could very well be generalized.