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>

modules/setting/setting.go

@@ -367,9 +367,8 @@ var (
 	}
 
 	// I18n settings
-	Langs     []string
-	Names     []string
-	dateLangs map[string]string
+	Langs []string
+	Names []string
 
 	// Highlight settings are loaded in modules/template/highlight.go
 
@@ -394,15 +393,6 @@ var (
 	UILocation = time.Local
 )
 
-// DateLang transforms standard language locale name to corresponding value in datetime plugin.
-func DateLang(lang string) string {
-	name, ok := dateLangs[lang]
-	if ok {
-		return name
-	}
-	return "en"
-}
-
 func getAppPath() (string, error) {
 	var appPath string
 	var err error
@@ -1012,7 +1002,6 @@ func NewContext() {
 			"español", "português do Brasil", "Português de Portugal", "polski", "български",
 			"italiano", "suomi", "Türkçe", "čeština", "српски", "svenska", "한국어"}
 	}
-	dateLangs = Cfg.Section("i18n.datelang").KeysHash()
 
 	ShowFooterBranding = Cfg.Section("other").Key("SHOW_FOOTER_BRANDING").MustBool(false)
 	ShowFooterVersion = Cfg.Section("other").Key("SHOW_FOOTER_VERSION").MustBool(true)