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/user/settings/security/openid.tmpl

@@ -2,19 +2,19 @@
 	{{ctx.Locale.Tr "settings.manage_openid"}}
 </h4>
 <div class="ui attached segment">
-	<div class="flex-list">
-		<div class="flex-item">
+	<div class="flex-divided-list items-with-main">
+		<div class="item">
 			{{ctx.Locale.Tr "settings.openid_desc"}}
 		</div>
 		{{range .OpenIDs}}
-			<div class="flex-item tw-items-center">
-				<div class="flex-item-leading">
+			<div class="item tw-items-center">
+				<div class="item-leading">
 					{{svg "fontawesome-openid" 20}}
 				</div>
-				<div class="flex-item-main">
-					<div class="flex-item-title">{{.URI}}</div>
+				<div class="item-main">
+					<div class="item-title">{{.URI}}</div>
 				</div>
-				<div class="flex-item-trailing">
+				<div class="item-trailing">
 					<form action="{{AppSubUrl}}/user/settings/security/openid/toggle_visibility" method="post">
 						<input name="id" type="hidden" value="{{.ID}}">
 						{{if .Show}}