Automation of end-date based on start-date

Hey,
@nursix Please Review
@flavour Removed the seperate js file and put in s3.datepicker.js
What I mean is that the user shouldn't see start date and end date, but start date and interval as that are the parameters he normally has at hand.

The "interval" would be a drop down with options "6 months", "1 year", "2 years", "5 years" - plus an additional option "Set End Date" which allows to enter a concrete end date if necessary. There should probably also be an option "permanent" which means "no end date".

I'm saying that because I think that typically, the standard period of validity of credentials does not differ between deployments, but between a) credential types and b) organisations, so that it is actually not possible - or at least futile - to define a useful site-wide standard period.

And yet UX can be significantly improved by a widget calculating the end date from start date and time interval instead of letting the user calculate the end date and then enter it. It's a computer, after all - so it shouldn't delegate computing tasks to the other side of the screen, and this one here can be anticipated.

I think deployment_settings are not always the right solution for certain problems - and this one is IMHO better solved with a widget.