Redmine

In #19468 was implemented the browser native date picker (when it's available) which is great for mobile devices, but to be honest, not so great when we speak about desktops/laptops. Being a project management/issue tracking platform, I expect that most of the usages are made from desktops/laptops (or at least, this is the case at Zitec).

I would like to propose to change a little bit the current behaviour and to use the native date picker only for mobile devices and for non-mobile devices, to use the custom date picker.
I made some tests using FlatPickr:"https://github.com/flatpickr/flatpickr" library and it works very well.

Why FlatPickr and not jQuery UI Date Picker?
- first of all, because the switch between native and non native is made by this library and we don't need to code it
- second of all, it is written in JS vanilla without any dependency, it is very light weight and has tests
- and finally, it offers a bunch of other really great features that can be useful for other improvements or by the plugins (for ex: time picker, range selections, etc).

At the same time, jQuery UI is not actively maintained (last release in September 14, 2016) and I think it is a good idea to start replace it with other modern libraries.

I'm attaching a very basic patch for demo and test purposes and to take some feedback, especially from Plan.io guys who were behind those two great additions (mobile responsive and native pickers).

Screenshots:
1. Non mobile devices:

2. Mobile devices: