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](f496457f-6afa-44be-a1e7-249ee5fe0706)

![image](c03b14f5-063d-4e13-9780-76ab002d76a9)

![image](f4b34e28-1546-4374-9199-c43348844edd)

---------

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

web_src/js/components/DiffCommitSelector.vue

@@ -247,6 +247,7 @@ export default {
             <div class="gt-ellipsis text light-2">
               {{ commit.committer_or_author_name }}
               <span class="text right">
+                <!-- TODO: make this respect the PreferredTimestampTense setting -->
                 <relative-time class="time-since" prefix="" :datetime="commit.time" data-tooltip-content data-tooltip-interactive="true">{{ commit.time }}</relative-time>
               </span>
             </div>