This PR introduces a new UI element type for Gitea called `flex-item`.
It consists of a horizontal card with a leading, main and trailing part:

The idea behind it is that in Gitea UI, we have many cases where we use
this kind of layout, but it is achieved in many different ways:
- grid layout
- `.ui.list` with additional hacky flexbox
- `.ui.key.list` - looks to me like a style set originally created for
ssh/gpg key list, was used in many other places
- `.issue.list` - created for issue cards, used in many other places
- ...
This new style is based on `.issue.list`, specifically the refactoring
of it done in #25750.
In this PR, the new element is introduced and lots of templates are
being refactored to use that style. This allows to remove a lot of
page-specific css, makes many of the elements responsive or simply
provides a cleaner/better-looking way to present information.
A devtest section with the new style is also available.
<details>
<summary>Screenshots (left: before, right: after)</summary>



















</details>
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Co-authored-by: Giteabot <teabot@gitea.io>
Should look exactly like before for normal dividers. "Horizontal" ones
look better because they no longer use image backgrounds.
<img width="917" alt="Screenshot 2023-06-27 at 19 07 56"
src="d97d8dec-6859-44a8-85ba-e4549b4dd9df">
<img width="914" alt="Screenshot 2023-06-27 at 19 05 58"
src="8bf98544-2d82-4ebf-ac68-d6dc237bd6b2">
<img width="1246" alt="Screenshot 2023-06-27 at 19 00 42"
src="36a6bb21-6029-4f53-8bee-535f55c66fed">
<img width="344" alt="Screenshot 2023-06-27 at 18 58 15"
src="a9e70aee-8e6b-4ea1-9e93-19c9f96aec6e">
<img width="823" alt="Screenshot 2023-06-27 at 18 56 22"
src="e7a497cd-f262-4683-8872-23c3c8cce32f">
<img width="330" alt="Screenshot 2023-06-27 at 19 21 11"
src="42f24149-a655-4c7e-bd26-8ab52db6446b">
Address
https://github.com/go-gitea/gitea/pull/25163#issuecomment-1599207916
Remove the unused "icon-button".
And fix the layout:
Without the dropdown icon:
```
{{svg "gitea-whitespace"}}
```

With the dropdown icon:
```
{{svg "gitea-whitespace" 16 "gt-mr-3"}}
{{svg "octicon-triangle-down" 14 "dropdown icon"}}
```

This refactors the `shared/datetime/short|long|full` templates into a
template helper function, which allows us to render absolute date times
within translatable phrases.
- Follows #23988
- The first attempt was in #24055
- This should help #22664
Changes:
1. Added the `DateTime` template helper that replaces the
`shared/datetime/short|long|full` templates
2. Used find-and-replace with varying regexes to replace the templates
from step 1 (for example, `\{\{template "shared/datetime/(\S+) \(dict
"Datetime" ([^"]+) "Fallback" ([^\)]+\)?) ?\)?\}\}` -> `{{DateTime "$1
$2 $3}}`)
3. Used the new `DateTime` helper in the issue due date timestamp
rendering
# Before

# After

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Signed-off-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>