Reduce margins on user settings page, introduce flex-container (#26046)

Same as https://github.com/go-gitea/gitea/pull/26026 but for the user
settings page. It introduces a new `flex-container` class and shares it
across both pages.

Before and After:
<img width="1264" alt="Screenshot 2023-07-21 at 19 35 57"
src="1358dab4-55c0-40ce-a4d5-673099304f3d">
<img width="1269" alt="Screenshot 2023-07-21 at 19 35 42"
src="34812f6d-dc65-4009-b977-90e03efdc6d1">

templates/admin/layout_head.tmpl

@@ -3,9 +3,9 @@
 	<div class="ui container">
 		{{template "base/alert" .ctxData}}
 	</div>
-	<div class="ui container admin-container">
+	<div class="ui container flex-container">
 		{{template "admin/navbar" .ctxData}}
-		<div class="admin-main">
+		<div class="flex-container-main">
 			{{/* block: admin-setting-content */}}
 
 {{if false}}{{/* to make html structure "likely" complete to prevent IDE warnings */}}

templates/admin/navbar.tmpl

@@ -1,4 +1,4 @@
-<div class="admin-nav">
+<div class="flex-container-nav">
 	<div class="ui fluid vertical menu">
 		<div class="header item">{{.locale.Tr "settings"}}</div>
 		<a class="{{if .PageIsAdminDashboard}}active {{end}}item" href="{{AppSubUrl}}/admin">