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 <me@silverwind.io>

templates/repo/issue/labels/edit_delete_label.tmpl

@@ -1,4 +1,4 @@
-<div class="ui gitea-confirm-modal delete modal">
+<div class="ui g-modal-confirm delete modal">
 	<div class="header">
 		{{svg "octicon-trash"}}
 		{{.locale.Tr "repo.issues.label_deletion"}}