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/repo/issue/view_content/sidebar.tmpl

@@ -385,7 +385,7 @@
 					<div class="gt-df gt-sb gt-ac">
 						<div class="due-date {{if .Issue.IsOverdue}}text red{{end}}" {{if .Issue.IsOverdue}}data-tooltip-content="{{.locale.Tr "repo.issues.due_date_overdue"}}"{{end}}>
 							{{svg "octicon-calendar" 16 "gt-mr-3"}}
-							<time data-format="date" datetime="{{.Issue.DeadlineUnix.FormatDate}}">{{.Issue.DeadlineUnix.FormatDate}}</time>
+							{{template "shared/datetime/long" (dict "Datetime" .Issue.DeadlineUnix.FormatDate "Fallback" .Issue.DeadlineUnix.FormatDate)}}
 						</div>
 						<div>
 							{{if and .HasIssuesOrPullsWritePermission (not .Repository.IsArchived)}}