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.

---

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>

services/context/context.go

@@ -121,6 +121,18 @@ func NewWebContext(base *Base, render Render, session session.Store) *Context {
 	return ctx
 }
 
+func (ctx *Context) AddPluralStringsToPageData(keys []string) {
+	for _, key := range keys {
+		array, fallback := ctx.Locale.TrPluralStringAllForms(key)
+
+		ctx.PageData["PLURALSTRINGS_LANG"].(map[string][]string)[key] = array
+
+		if fallback != nil {
+			ctx.PageData["PLURALSTRINGS_FALLBACK"].(map[string][]string)[key] = fallback
+		}
+	}
+}
+
 // Contexter initializes a classic context for a request.
 func Contexter() func(next http.Handler) http.Handler {
 	rnd := templates.HTMLRenderer()
@@ -208,6 +220,25 @@ func Contexter() func(next http.Handler) http.Handler {
 
 			ctx.Data["AllLangs"] = translation.AllLangs()
 
+			ctx.PageData["PLURAL_RULE_LANG"] = translation.GetPluralRule(ctx.Locale)
+			ctx.PageData["PLURAL_RULE_FALLBACK"] = translation.GetDefaultPluralRule()
+			ctx.PageData["PLURALSTRINGS_LANG"] = map[string][]string{}
+			ctx.PageData["PLURALSTRINGS_FALLBACK"] = map[string][]string{}
+
+			ctx.AddPluralStringsToPageData([]string{"relativetime.mins", "relativetime.hours", "relativetime.days", "relativetime.weeks", "relativetime.months", "relativetime.years"})
+
+			ctx.PageData["DATETIMESTRINGS"] = map[string]string{
+				"FUTURE": ctx.Locale.TrString("relativetime.future"),
+				"NOW":    ctx.Locale.TrString("relativetime.now"),
+			}
+			for _, key := range []string{"relativetime.1day", "relativetime.1week", "relativetime.1month", "relativetime.1year", "relativetime.2days", "relativetime.2weeks", "relativetime.2months", "relativetime.2years"} {
+				// These keys are used for special-casing some time words. We only add keys that are actually translated, so that we
+				// can fall back to the generic pluralized time word in the correct language if the special case is untranslated.
+				if ctx.Locale.HasKey(key) {
+					ctx.PageData["DATETIMESTRINGS"].(map[string]string)[key] = ctx.Locale.TrString(key)
+				}
+			}
+
 			next.ServeHTTP(ctx.Resp, ctx.Req)
 		})
 	}