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>

options/locale_next/locale_en-US.json

@@ -4,6 +4,40 @@
     "home.explore_repos": "Explore repositories",
     "home.explore_users": "Explore users",
     "home.explore_orgs": "Explore organizations",
+    "relativetime.now": "now",
+    "relativetime.future": "in future",
+    "relativetime.mins": {
+        "one": "%d minute ago",
+        "other": "%d minutes ago"
+    },
+    "relativetime.hours": {
+        "one": "%d hour ago",
+        "other": "%d hours ago"
+    },
+    "relativetime.days": {
+        "one": "%d day ago",
+        "other": "%d days ago"
+    },
+    "relativetime.weeks": {
+        "one": "%d week ago",
+        "other": "%d weeks ago"
+    },
+    "relativetime.months": {
+        "one": "%d month ago",
+        "other": "%d months ago"
+    },
+    "relativetime.years": {
+        "one": "%d year ago",
+        "other": "%d years ago"
+    },
+    "relativetime.1day": "yesterday",
+    "relativetime.2days": "two days ago",
+    "relativetime.1week": "last week",
+    "relativetime.2weeks": "two weeks ago",
+    "relativetime.1month": "last month",
+    "relativetime.2months": "two months ago",
+    "relativetime.1year": "last year",
+    "relativetime.2years": "two years ago",
     "repo.pulls.merged_title_desc": {
         "one": "merged %[1]d commit from <code>%[2]s</code> into <code>%[3]s</code> %[4]s",
         "other": "merged %[1]d commits from <code>%[2]s</code> into <code>%[3]s</code> %[4]s"