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>

docs/content/administration/config-cheat-sheet.en-us.md

@@ -231,6 +231,7 @@ The following configuration set `Content-Type: application/vnd.android.package-a
 - `ONLY_SHOW_RELEVANT_REPOS`: **false**: Whether to only show relevant repos on the explore page when no keyword is specified and default sorting is used.
     A repo is considered irrelevant if it's a fork or if it has no metadata (no description, no icon, no topic).
 - `EXPLORE_PAGING_DEFAULT_SORT`: **recentupdate**: Change the sort type of the explore pages. Valid values are "recentupdate", "alphabetically", "reverselastlogin", "newest" and "oldest"
+- `PREFERRED_TIMESTAMP_TENSE`: **mixed**: The tense all timestamps should be rendered in. Possible values are `absolute` time (i.e. 1970-01-01, 11:59) and `mixed`. `mixed` means most timestamps are rendered in relative time (i.e. 2 days ago).
 
 ### UI - Admin (`ui.admin`)