Use auto-updating, natively hoverable, localized time elements (#23988)

- Added [GitHub's `relative-time` element](https://github.com/github/relative-time-element)
- Converted all formatted timestamps to use this element
- No more flashes of unstyled content around time elements
- These elements are localized using the `lang` property of the HTML file
- Relative (e.g. the activities in the dashboard) and duration (e.g.
server uptime in the admin page) time elements are auto-updated to keep
up with the current time without refreshing the page
- Code that is not needed anymore such as `formatting.js` and parts of `since.go` have been deleted

Replaces #21440
Follows #22861

## Screenshots

### Localized

![image](https://user-images.githubusercontent.com/20454870/230775041-f0af4fda-8f6b-46d3-b8e3-d340c791a50c.png)

![image](https://user-images.githubusercontent.com/20454870/230673393-931415a9-5729-4ac3-9a89-c0fb5fbeeeb7.png)

### Tooltips

#### Native for dates

![image](https://user-images.githubusercontent.com/20454870/230797525-1fa0a854-83e3-484c-9da5-9425ab6528a3.png)

#### Interactive for relative

![image](https://user-images.githubusercontent.com/115237/230796860-51e1d640-c820-4a34-ba2e-39087020626a.png)

### Auto-update

![rec](https://user-images.githubusercontent.com/20454870/230672159-37480d8f-435a-43e9-a2b0-44073351c805.gif)

---------

Signed-off-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: delvh <dev.lh@web.de>

templates/admin/cron.tmpl

@@ -21,8 +21,8 @@
 						<td><button type="submit" class="ui green button" name="op" value="{{.Name}}" title="{{$.locale.Tr "admin.dashboard.operation_run"}}">{{svg "octicon-triangle-right"}}</button></td>
 						<td>{{$.locale.Tr (printf "admin.dashboard.%s" .Name)}}</td>
 						<td>{{.Spec}}</td>
-						<td>{{DateFmtLong .Next}}</td>
-						<td>{{if gt .Prev.Year 1}}{{DateFmtLong .Prev}}{{else}}N/A{{end}}</td>
+						<td>{{template "shared/datetime/full" (dict "Datetime" (DateFmtLong .Next) "Fallback" (DateFmtLong .Next) )}}</td>
+						<td>{{if gt .Prev.Year 1}}{{template "shared/datetime/full" (dict "Datetime" (DateFmtLong .Prev) "Fallback" (DateFmtLong .Prev) )}}{{else}}N/A{{end}}</td>
 						<td>{{.ExecTimes}}</td>
 						<td {{if ne .Status ""}}data-tooltip-content="{{.FormatLastMessage $.locale}}"{{end}} >{{if eq .Status ""}}—{{else if eq .Status "finished"}}{{svg "octicon-check" 16}}{{else}}{{svg "octicon-x" 16}}{{end}}</td>
 					</tr>