Allow using localized absolute date times within phrases with place holders and localize issue due date events (#24275)

This refactors the `shared/datetime/short|long|full` templates into a
template helper function, which allows us to render absolute date times
within translatable phrases.

- Follows #23988
- The first attempt was in #24055
- This should help #22664 

Changes:
1. Added the `DateTime` template helper that replaces the
`shared/datetime/short|long|full` templates
2. Used find-and-replace with varying regexes to replace the templates
from step 1 (for example, `\{\{template "shared/datetime/(\S+) \(dict
"Datetime" ([^"]+) "Fallback" ([^\)]+\)?) ?\)?\}\}` -> `{{DateTime "$1
$2 $3}}`)
3. Used the new `DateTime` helper in the issue due date timestamp
rendering

# Before

![image](https://user-images.githubusercontent.com/20454870/233791256-b454c455-aca0-4b76-b300-7866c7bd529e.png)

# After

![image](https://user-images.githubusercontent.com/20454870/233790809-c4913355-2822-4657-bb29-2298deb6d4b3.png)

---------

Signed-off-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/timeutil/datetime.go

@@ -0,0 +1,25 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package timeutil
+
+import (
+	"fmt"
+	"html"
+	"html/template"
+)
+
+// DateTime renders an absolute time HTML given a time as a string
+func DateTime(format, datetime, fallback string) template.HTML {
+	datetimeEscaped := html.EscapeString(datetime)
+	fallbackEscaped := html.EscapeString(fallback)
+	switch format {
+	case "short":
+		return template.HTML(fmt.Sprintf(`<relative-time format="datetime" year="numeric" month="short" day="numeric" weekday="" datetime="%s">%s</relative-time>`, datetimeEscaped, fallbackEscaped))
+	case "long":
+		return template.HTML(fmt.Sprintf(`<relative-time format="datetime" year="numeric" month="long" day="numeric" weekday="" datetime="%s">%s</relative-time>`, datetimeEscaped, fallbackEscaped))
+	case "full":
+		return template.HTML(fmt.Sprintf(`<relative-time format="datetime" weekday="" year="numeric" month="short" day="numeric" hour="numeric" minute="numeric" second="numeric" datetime="%s">%s</relative-time>`, datetimeEscaped, fallbackEscaped))
+	}
+	return template.HTML("error in DateTime")
+}