Refactor "flex-list" to "flex-divided-list" (#37505)

Purpose:

1. Make the whole code base have unified "item" layout
2. Clarify our "list" styles: "flex-relaxed-list", "flex-divided-list"
3. Prepare to replace legacy "ui relaxed list"
* https://github.com/go-gitea/gitea/pull/37445#discussion_r3144458865
4. Prepare for refactoring the "pull merge box", it needs the
"flex-divided-list"
    * related to "Refactor pull request view (*)" like #37451
5. Fix legacy abuses of "flex-list", e.g.: repo home sidebar

templates/shared/secrets/add_list.tmpl

@@ -16,24 +16,24 @@
 </h4>
 <div class="ui attached segment">
 	{{if .Secrets}}
-	<div class="flex-list">
+	<div class="flex-divided-list items-with-main">
 		{{range .Secrets}}
-		<div class="flex-item tw-items-center">
-			<div class="flex-item-leading">
+		<div class="item tw-items-center">
+			<div class="item-leading">
 				{{svg "octicon-key" 32}}
 			</div>
-			<div class="flex-item-main">
-				<div class="flex-item-title">
+			<div class="item-main">
+				<div class="item-title">
 					{{.Name}}
 				</div>
-				<div class="flex-item-body">
+				<div class="item-body">
 					{{if .Description}}{{.Description}}{{else}}-{{end}}
 				</div>
-				<div class="flex-item-body">
+				<div class="item-body">
 					******
 				</div>
 			</div>
-			<div class="flex-item-trailing">
+			<div class="item-trailing">
 				<span class="color-text-light-2">
 					{{ctx.Locale.Tr "settings.added_on" (DateUtils.AbsoluteShort .CreatedUnix)}}
 				</span>