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/admin/user/view_emails.tmpl

@@ -1,7 +1,7 @@
-<div class="flex-list">
+<div class="flex-divided-list items-with-main">
 	{{range .Emails}}
-		<div class="flex-item">
-			<div class="flex-item-main">
+		<div class="item">
+			<div class="item-main">
 				<div class="flex-text-block">
 					<a href="mailto:{{.Email}}">{{.Email}}</a>
 					{{if .IsPrimary}}