Re-add calendar dropdown for selecting dates

[femtobit]

The current date selection field inteprets inputs like "in 2 weeks" but does not show a calendar to allow users to select dates visually, which is useful especially in cases where the user is not certain about the exact dates they wish to include in the poll and needs a visual overview in order to decide.

I would therefore suggest adding a dropdown calendar like in previous versions.

[opatut]

I always found the calendar popup hard to use and always in the way. It was hard to use with keyboard only, hid other parts of the form, and was not styled at all. That's why I removed it. Also I think, since you can use "friday next week" etc, you don't need to have the overview, and if you do, I'm sure calendar.google.com can help.

I am open to suggestions though, if you have a nice idea how to display the calendar, but make it not annoying, I'd be fine with inserting one.