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)

---

## 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/mock.go

@@ -6,11 +6,15 @@ package translation
 import (
 	"fmt"
 	"html/template"
+
+	"forgejo.org/modules/translation/i18n"
 )
 
-// MockLocale provides a mocked locale without any translations
+// MockLocale provides a mocked locale without any translations, other than those inserted into MockTranslations by a testcase
 type MockLocale struct {
 	Lang, LangName string // these fields are used directly in templates: ctx.Locale.Lang
+
+	MockTranslations map[string]string
 }
 
 var _ Locale = (*MockLocale)(nil)
@@ -20,11 +24,14 @@ func (l MockLocale) Language() string {
 }
 
 func (l MockLocale) TrString(s string, _ ...any) string {
+	if val, ok := l.MockTranslations[s]; ok {
+		return val
+	}
 	return s
 }
 
 func (l MockLocale) Tr(s string, a ...any) template.HTML {
-	return template.HTML(s)
+	return template.HTML(l.TrString(s))
 }
 
 func (l MockLocale) TrN(cnt any, key1, keyN string, args ...any) template.HTML {
@@ -35,6 +42,11 @@ func (l MockLocale) TrPluralString(count any, trKey string, trArgs ...any) templ
 	return template.HTML(trKey)
 }
 
+// TrPluralStringAllForms implements Locale.
+func (l MockLocale) TrPluralStringAllForms(trKey string) ([]string, []string) {
+	return []string{l.TrString(trKey + i18n.PluralFormSeparator + "one"), l.TrString(trKey + i18n.PluralFormSeparator + "other")}, nil
+}
+
 func (l MockLocale) TrSize(s int64) ReadableSize {
 	return ReadableSize{fmt.Sprint(s), ""}
 }