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/plural_rules.go

@@ -251,3 +251,34 @@ var PluralRules = []i18n.PluralFormRule{
 		return i18n.PluralFormOther
 	},
 }
+
+var UsedPluralForms = [][]i18n.PluralFormIndex{
+	// [ 0] Common 2-form, e.g. English, German
+	{i18n.PluralFormOne, i18n.PluralFormOther},
+	// [ 1] Bengali
+	{i18n.PluralFormOne, i18n.PluralFormOther},
+	// [ 2] Icelandic
+	{i18n.PluralFormOne, i18n.PluralFormOther},
+	// [ 3] Filipino
+	{i18n.PluralFormOne, i18n.PluralFormOther},
+	// [ 4] OneForm
+	{i18n.PluralFormOther},
+	// [ 5] Czech
+	{i18n.PluralFormOne, i18n.PluralFormFew, i18n.PluralFormOther},
+	// [ 6] Russian
+	{i18n.PluralFormOne, i18n.PluralFormFew, i18n.PluralFormMany},
+	// [ 7] Polish
+	{i18n.PluralFormOne, i18n.PluralFormFew, i18n.PluralFormOther},
+	// [ 8] Latvian
+	{i18n.PluralFormZero, i18n.PluralFormOne, i18n.PluralFormOther},
+	// [ 9] Lithuanian
+	{i18n.PluralFormOne, i18n.PluralFormFew, i18n.PluralFormMany},
+	// [10] French
+	{i18n.PluralFormOne, i18n.PluralFormMany, i18n.PluralFormOther},
+	// [11] Catalan
+	{i18n.PluralFormOne, i18n.PluralFormMany, i18n.PluralFormOther},
+	// [12] Slovenian
+	{i18n.PluralFormOne, i18n.PluralFormTwo, i18n.PluralFormFew, i18n.PluralFormOther},
+	// [13] Arabic
+	{i18n.PluralFormZero, i18n.PluralFormOne, i18n.PluralFormTwo, i18n.PluralFormFew, i18n.PluralFormMany, i18n.PluralFormOther},
+}