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>

web_src/css/helpers.css

@@ -1,3 +1,7 @@
+/*
+Gitea's tailwind-style CSS helper classes have `gt-` prefix.
+Gitea's private styles use `g-` prefix.
+*/
 .gt-df { display: flex !important; }
 .gt-di { display: inline !important; }
 .gt-dif { display: inline-flex !important; }

web_src/css/modules/modal.css

@@ -1,8 +1,15 @@
-.ui.modal.gitea-confirm-modal {
+.ui.modal.g-modal-confirm {
   max-width: min(800px, 90vw);
   width: fit-content;
 }
 
+.ui.modal.g-modal-confirm > .inside.close {
+  padding: 0;
+  width: 1em;
+  height: 1em;
+  top: 1.2em;
+}
+
 .ui.modal > .header {
   /* can't use display:flex, because some headers have space-separated elements, eg: delete branch modal */
   color: var(--color-text-dark);
@@ -42,6 +49,13 @@
   padding: 10px 12px 10px 10px;
 }
 
+.ui.modal .actions > .ui.button.danger {
+  display: block;
+  width: 100%;
+  margin: 0 auto;
+  text-align: center;
+}
+
 .ui.modal .actions > .ui.button .svg {
   margin-right: 5px;
 }