Fixes https://github.com/go-gitea/gitea/issues/30082.
Adds a new linter that searches for non-existant SVG images in
templates. Output before the fix was:
```
$ make lint-templates
SVG "octicon-warning" not found, used in templates/devtest/flex-list.tmpl
SVG "octicon-warning" not found, used in templates/devtest/flex-list.tmpl
SVG "octicon-markup" not found, used in templates/repo/diff/comment_form.tmpl
make: *** [Makefile:438: lint-templates] Error 1
```
<img width="306" alt="Screenshot 2024-03-25 at 23 31 05"
src="https://github.com/go-gitea/gitea/assets/115237/1052d1a9-bfec-4d5a-9cae-f895f78f7c93">
Replace #26761
It's better to keep children elements simple, and let parent containers
layout the necessary padding/margin.
The old `not(:last-child)` and `.flex-item + .flex-item` are not easy to
maintain (for example, what if the developer would like to use a "tiny
height" item?)
The old approach also makes some UI look strange because the first item
doesn't have proper padding-top.
In this PR, we just simply use `.flex-item { padding: ... }`:
* Developers could manually set the item height they want easily
* It's easier to make it work with various containers -- with padding
(`ui segment`) and without padding (`div`)
And added more samples/examples.
Co-authored-by: Giteabot <teabot@gitea.io>
Fix #26617
1. Separate the "flex-list" examples into a dedicated template, and add some more examples
2. Use `flex-basis` instead of `flex-shrink` for `flex-item-trailing`, to avoid wrapping the texts too aggressively
3. Some `flex-wrap: wrap;` are removed
