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/shared/datetime/full.tmpl

@@ -0,0 +1 @@
+<relative-time format="datetime" weekday="" year="numeric" month="short" day="numeric" hour="numeric" minute="numeric" second="numeric" datetime="{{.Datetime}}">{{.Fallback}}</relative-time>

templates/shared/datetime/long.tmpl

@@ -0,0 +1 @@
+<relative-time format="datetime" year="numeric" month="long" day="numeric" weekday="" datetime="{{.Datetime}}">{{.Fallback}}</relative-time>

templates/shared/datetime/short.tmpl

@@ -0,0 +1 @@
+<relative-time format="datetime" year="numeric" month="short" day="numeric" weekday="" datetime="{{.Datetime}}">{{.Fallback}}</relative-time>

templates/shared/issuelist.tmpl

@@ -106,7 +106,7 @@
 						<span class="due-date" data-tooltip-content="{{$.locale.Tr "repo.issues.due_date"}}">
 							<span{{if .IsOverdue}} class="overdue"{{end}}>
 								{{svg "octicon-calendar" 14 "gt-mr-2"}}
-								<time data-format="short-date" datetime="{{.DeadlineUnix.FormatDate}}">{{.DeadlineUnix.FormatShort}}</time>
+								{{template "shared/datetime/short" (dict "Datetime" .DeadlineUnix.FormatDate "Fallback" .DeadlineUnix.FormatShort)}}
 							</span>
 						</span>
 					{{end}}