Replace the 'relative-time' element scripting with custom, translatable rewrite (#6154)

This is my take to fix #6078
Should also resolve #6111

As far as I can tell, Forgejo uses only a subset of the relative-time functionality, and as far as I can see, this subset can be implemented using browser built-in date conversion and arithmetic. So I wrote a JavaScript to format the relative-time element accordingly, and a Go binding to generate the translated elements.

This is my first time writing Go code, and my first time coding for a large-scale server application, so please tell me if I'm doing something wrong, or if the whole approach is not acceptable.

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Screenshot: Localized times in Low German
![grafik](/attachments/6f787e17-e666-4b88-8599-af0b8357ffbe)
Screenshot: The same with Forgejo in English
![grafik](/attachments/af09c873-b9f3-423d-b12b-7e62093e2623)

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## Checklist

The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).

### Tests

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [x] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6154
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Co-authored-by: Benedikt Straub <benedikt-straub@web.de>
Co-committed-by: Benedikt Straub <benedikt-straub@web.de>

modules/translation/translation.go

@@ -46,6 +46,8 @@ type Locale interface {
 	HasKey(trKey string) bool
 
 	PrettyNumber(v any) string
+
+	TrPluralStringAllForms(trKey string) ([]string, []string)
 }
 
 // LangType represents a lang type
@@ -108,8 +110,9 @@ func InitLocales(ctx context.Context) {
 				}
 			}
 
+			pluralRuleIndex := GetPluralRuleImpl(setting.Langs[i])
 			key := "locale_" + setting.Langs[i] + ".ini"
-			if err = i18n.DefaultLocales.AddLocaleByIni(setting.Langs[i], setting.Names[i], PluralRules[GetPluralRuleImpl(setting.Langs[i])], localeDataBase, localeData[key]); err != nil {
+			if err = i18n.DefaultLocales.AddLocaleByIni(setting.Langs[i], setting.Names[i], PluralRules[pluralRuleIndex], UsedPluralForms[pluralRuleIndex], localeDataBase, localeData[key]); err != nil {
 				log.Error("Failed to set old-style messages to %s: %v", setting.Langs[i], err)
 			}
 
@@ -324,6 +327,14 @@ func (l *locale) PrettyNumber(v any) string {
 	return l.msgPrinter.Sprintf("%v", number.Decimal(v))
 }
 
+func GetPluralRule(l Locale) int {
+	return GetPluralRuleImpl(l.Language())
+}
+
+func GetDefaultPluralRule() int {
+	return GetPluralRuleImpl(i18n.DefaultLocales.GetDefaultLang())
+}
+
 func init() {
 	// prepare a default matcher, especially for tests
 	supportedTags = []language.Tag{language.English}