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>

custom/conf/app.ini.sample

@@ -17,7 +17,7 @@ SCRIPT_TYPE = bash
 ; DETECTED_CHARSETS_ORDER tie-break order for detected charsets.
 ; If the charsets have equal confidence, tie-breaking will be done by order in this list
 ; with charsets earlier in the list chosen in preference to those later.
-; Adding "defaults" will place the unused charsets at that position. 
+; Adding "defaults" will place the unused charsets at that position.
 DETECTED_CHARSETS_ORDER=UTF-8, UTF-16BE, UTF-16LE, UTF-32BE, UTF-32LE, ISO-8859, windows-1252, ISO-8859, windows-1250, ISO-8859, ISO-8859, ISO-8859, windows-1253, ISO-8859, windows-1255, ISO-8859, windows-1251, windows-1256, KOI8-R, ISO-8859, windows-1254, Shift_JIS, GB18030, EUC-JP, EUC-KR, Big5, ISO-2022, ISO-2022, ISO-2022, IBM424_rtl, IBM424_ltr, IBM420_rtl, IBM420_ltr
 ; Default ANSI charset to override non-UTF-8 charsets to
 ANSI_CHARSET =
@@ -634,8 +634,8 @@ SUBJECT_PREFIX =
 ; Mail server
 ; Gmail: smtp.gmail.com:587
 ; QQ: smtp.qq.com:465
-; Using STARTTLS on port 587 is recommended per RFC 6409. 
-; Note, if the port ends with "465", SMTPS will be used. 
+; Using STARTTLS on port 587 is recommended per RFC 6409.
+; Note, if the port ends with "465", SMTPS will be used.
 HOST =
 ; Disable HELO operation when hostnames are different.
 DISABLE_HELO =
@@ -950,32 +950,6 @@ MAX_TOKEN_LENGTH=32767
 LANGS = en-US,zh-CN,zh-HK,zh-TW,de-DE,fr-FR,nl-NL,lv-LV,ru-RU,uk-UA,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
 NAMES = English,简体中文,繁體中文（香港）,繁體中文（台灣）,Deutsch,français,Nederlands,latviešu,русский,Українська,日本語,español,português do Brasil,Português de Portugal,polski,български,italiano,suomi,Türkçe,čeština,српски,svenska,한국어
 
-; Used for datetimepicker
-[i18n.datelang]
-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
-uk-UA = uk
-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]
 ; NOTE: THE DEFAULT VALUES HERE WILL NEED TO BE CHANGED
 ; Two Factor authentication with security keys