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>

docs/content/page/index.zh-cn.md

@@ -57,7 +57,6 @@ Gitea的首要目标是创建一个极易安装，运行非常快速，安装和
   * [Highlight](https://highlightjs.org/)
   * [Clipboard](https://zenorocha.github.io/clipboard.js/)
   * [CodeMirror](https://codemirror.net/)
-  * [jQuery Date Time Picker](https://github.com/xdan/datetimepicker)
   * [jQuery MiniColors](https://github.com/claviska/jquery-minicolors)
 * 数据库驱动：
   * [github.com/go-sql-driver/mysql](https://github.com/go-sql-driver/mysql)