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/accountlinks.tmpl

@@ -21,25 +21,25 @@
 </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.manage_account_links_desc"}}
 		</div>
 		{{range $loginSource, $provider := .AccountLinks}}
-			<div class="flex-item">
+			<div class="item">
 				{{$providerData := index $.OAuth2Providers $loginSource.Name}}
-				<div class="flex-item-leading">
+				<div class="item-leading">
 					{{$providerData.IconHTML 20}}
 				</div>
-				<div class="flex-item-main">
-					<span class="flex-item-title" data-tooltip-content="{{$provider}}">
+				<div class="item-main">
+					<span class="item-title" data-tooltip-content="{{$provider}}">
 						{{$loginSource.Name}}
 					</span>
 					{{if $loginSource.IsActive}}
 					<span class="flex-text-body tw-text-primary">{{ctx.Locale.Tr "repo.settings.active"}}</span>
 					{{end}}
 				</div>
-				<div class="flex-item-trailing">
+				<div class="item-trailing">
 					<button class="ui red tiny button delete-button" data-modal-id="delete-account-link" data-url="{{AppSubUrl}}/user/settings/security/account_link" data-id="{{$loginSource.ID}}">
 						{{ctx.Locale.Tr "settings.delete_key"}}
 					</button>