Use new datetimepicker in Talks

[Splines]

Apparently, we are still using yet another datetimepicker in the talks page. #526 aimed to replace the old datetimepicker but forgot this.

[fosterfarrell9]

Note that we need a datepicker here, not a datetimepicker. Currently the corresponding partial uses the datepicker provided by the browser, not needing any extra js (that's why it was probably not seen when the update was made). Browser support for datepicker is not ideal, but not so bad either. But of course, if our datetimepicker has an option to be reduced to be a datepicker, that would be nice.