Improve some modal action buttons (#24289)

Follow #24097 and #24285

And add a devtest page for modal action button testing.
http://localhost:3000/devtest/fomantic-modal

Now the `modal_actions_confirm.tmpl` could support: green / blue /
yellow positive buttons, the negative button is "secondary".

ps: this PR is only a small improvement, there are still a lot of
buttons not having proper colors. In the future these buttons could be
improved by this approach.

These buttons could also be improved according to the conclusion of
#24285 in the future.



![image](https://user-images.githubusercontent.com/2114189/233847773-a6d6b29b-7b5c-490e-8425-40dfd0ad2529.png)


And add GitHub-like single danger button (context:
https://github.com/go-gitea/gitea/issues/24285#issuecomment-1519100312)


![image](https://user-images.githubusercontent.com/2114189/233891566-055d7611-894d-4d5a-baf5-f6369180bf8d.png)

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Co-authored-by: silverwind <[email protected]>

doc/contributing/guidelines-frontend.en-us.md

@@ -49,6 +49,7 @@ We recommend [Google HTML/CSS Style Guide](https://google.github.io/styleguide/h
 9. Avoid unnecessary `!important` in CSS, add comments to explain why it's necessary if it can't be avoided.
 10. Avoid mixing different events in one event listener, prefer to use individual event listeners for every event.
 11. Custom event names are recommended to use `ce-` prefix.
+12. Gitea's tailwind-style CSS classes use `gt-` prefix (`gt-relative`), while Gitea's own private framework-level CSS classes use `g-` prefix (`g-modal-confirm`).
 
 ### Accessibility / ARIA