Replace jquery-datetimepicker with native date input (#11684)

This removes the jQuery plugin as well as the associated config options.
Native input[type=date] does not require a language attribute as it is
localized by default, except for the placeholder attribute for which I
currently piggy-back the repo.issues.due_date_form localization option.

Implementation should pretty much match GH. Of note is that Safari does
not provide a UI for this input type, but I don't think providing one is
neccessary and GH did not bother either.

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

web_src/js/features/datetimepicker.js

@@ -1,8 +0,0 @@
-export default async function initDateTimePicker(locale) {
-  await Promise.all([
-    import(/* webpackChunkName: "datetimepicker" */'jquery-datetimepicker'),
-    import(/* webpackChunkName: "datetimepicker" */'jquery-datetimepicker/build/jquery.datetimepicker.min.css'),
-  ]);
-
-  $.datetimepicker.setLocale(locale);
-}