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/organization.tmpl

@@ -10,19 +10,19 @@
 		</h4>
 		<div class="ui attached segment orgs">
 			{{if .Orgs}}
-				<div class="flex-list">
+				<div class="flex-divided-list items-with-main">
 					{{range .Orgs}}
-					<div class="flex-item">
-						<div class="flex-item-leading">
+					<div class="item">
+						<div class="item-leading">
 							{{ctx.AvatarUtils.Avatar . 28 "mini"}}
 						</div>
-						<div class="flex-item-main">
-							<div class="flex-item-title">{{template "shared/user/name" .}}</div>
+						<div class="item-main">
+							<div class="item-title">{{template "shared/user/name" .}}</div>
 							<div class="flex-text-body">
 								{{.Description}}
 							</div>
 						</div>
-						<div class="flex-item-trailing">
+						<div class="item-trailing">
 							<form>
 								<button class="ui red button delete-button" data-modal-id="leave-organization"
 												data-url="{{.OrganisationLink}}/members/action/leave" data-datauid="{{$.SignedUser.ID}}"