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)

---------

Co-authored-by: silverwind <me@silverwind.io>

templates/repo/issue/view_content/sidebar.tmpl

@@ -542,7 +542,7 @@
 			{{if and .CanCreateIssueDependencies (not .Repository.IsArchived)}}
 				<input type="hidden" id="crossRepoSearch" value="{{.AllowCrossRepositoryDependencies}}">
 
-				<div class="ui gitea-confirm-modal modal remove-dependency">
+				<div class="ui g-modal-confirm modal remove-dependency">
 					<div class="header">
 						{{svg "octicon-trash"}}
 						{{.locale.Tr "repo.issues.dependency.remove_header"}}
@@ -656,7 +656,7 @@
 				{{svg "octicon-trash"}}
 				{{.locale.Tr "repo.issues.delete"}}
 			</button>
-			<div class="ui gitea-confirm-modal modal" id="delete">
+			<div class="ui g-modal-confirm modal" id="delete">
 				<div class="header">
 					{{if .Issue.IsPull}}
 						{{.locale.Tr "repo.pulls.delete.title"}}