Add global setting how timestamps should be rendered (#28657)

- Resolves https://github.com/go-gitea/gitea/issues/22493
- Related to https://github.com/go-gitea/gitea/issues/4520

Some admins prefer all timestamps to display the full date instead of
relative time. They can do that now by setting

```ini
[ui]
PREFERRED_TIMESTAMP_TENSE = absolute
```

This setting is set to `mixed` by default, allowing dates to render as
"5 hours ago". Here are some screenshots of the UI with this setting set
to `absolute`:

![image](https://github.com/go-gitea/gitea/assets/20454870/f496457f-6afa-44be-a1e7-249ee5fe0706)

![image](https://github.com/go-gitea/gitea/assets/20454870/c03b14f5-063d-4e13-9780-76ab002d76a9)

![image](https://github.com/go-gitea/gitea/assets/20454870/f4b34e28-1546-4374-9199-c43348844edd)

---------

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

modules/timeutil/since.go

@@ -9,6 +9,7 @@ import (
 	"strings"
 	"time"
 
+	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/translation"
 )
 
@@ -132,6 +133,9 @@ func timeSinceUnix(then, now time.Time, lang translation.Locale) template.HTML {
 
 // TimeSince renders relative time HTML given a time.Time
 func TimeSince(then time.Time, lang translation.Locale) template.HTML {
+	if setting.UI.PreferredTimestampTense == "absolute" {
+		return DateTime("full", then, `class="time-since"`)
+	}
 	return timeSinceUnix(then, time.Now(), lang)
 }