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/doc/advanced/config-cheat-sheet.en-us.md

@@ -614,32 +614,6 @@ NB: You must `REDIRECT_MACARON_LOG` and have `DISABLE_ROUTER_LOG` set to `false`
 - `LANGS`: **en-US,zh-CN,zh-HK,zh-TW,de-DE,fr-FR,nl-NL,lv-LV,ru-RU,ja-JP,es-ES,pt-BR,pt-PT,pl-PL,bg-BG,it-IT,fi-FI,tr-TR,cs-CZ,sr-SP,sv-SE,ko-KR**: List of locales shown in language selector
 - `NAMES`: **English,简体中文,繁體中文（香港）,繁體中文（台灣）,Deutsch,français,Nederlands,latviešu,русский,日本語,español,português do Brasil,Português de Portugal,polski,български,italiano,suomi,Türkçe,čeština,српски,svenska,한국어**: Visible names corresponding to the locales
 
-### i18n - Datepicker Language (`i18n.datelang`)
-Maps locales to the languages used by the datepicker plugin
-
-- `en-US`: **en**
-- `zh-CN`: **zh**
-- `zh-HK`: **zh-HK**
-- `zh-TW`: **zh-TW**
-- `de-DE`: **de**
-- `fr-FR`: **fr**
-- `nl-NL`: **nl**
-- `lv-LV`: **lv**
-- `ru-RU`: **ru**
-- `ja-JP`: **ja**
-- `es-ES`: **es**
-- `pt-BR`: **pt-BR**
-- `pt-PT`: **pt**
-- `pl-PL`: **pl**
-- `bg-BG`: **bg**
-- `it-IT`: **it**
-- `fi-FI`: **fi**
-- `tr-TR`: **tr**
-- `cs-CZ`: **cs-CZ**
-- `sr-SP`: **sr**
-- `sv-SE`: **sv**
-- `ko-KR`: **ko**
-
 ## U2F (`U2F`)
 - `APP_ID`: **`ROOT_URL`**: Declares the facet of the application. Requires HTTPS.
 - `TRUSTED_FACETS`: List of additional facets which are trusted. This is not support by all browsers.