Use flex-container for repo and org settings (#27418) (#27430)

Backport #27418 by @silverwind

Same as https://github.com/go-gitea/gitea/pull/26046 but for repo and
org settings pages, reducing the margins between the boxes:

<img width="1247" alt="Screenshot 2023-10-03 at 23 25 19"
src="https://github.com/go-gitea/gitea/assets/115237/4e68ad5e-5fdc-4466-aefb-ec71bf411d45">
<img width="1255" alt="Screenshot 2023-10-03 at 23 27 12"
src="https://github.com/go-gitea/gitea/assets/115237/9068369b-a75d-401e-8b8d-3bd4bbe097dc">

Co-authored-by: silverwind <me@silverwind.io>

templates/org/settings/layout_head.tmpl

@@ -1,9 +1,9 @@
 {{template "base/head" .ctxData}}
 <div role="main" aria-label="{{.ctxData.Title}}" class="page-content {{.pageClass}}">
 	{{template "org/header" .ctxData}}
-	<div class="ui container stackable grid">
+	<div class="ui container flex-container">
 		{{template "org/settings/navbar" .ctxData}}
-		<div class="twelve wide column">
+		<div class="flex-container-main">
 			{{template "base/alert" .ctxData}}
 			{{/* block: org-setting-content */}}
 

templates/org/settings/navbar.tmpl

@@ -1,4 +1,4 @@
-<div class="four wide column">
+<div class="flex-container-nav">
 	<div class="ui fluid vertical menu">
 		<div class="header item">{{ctx.Locale.Tr "org.settings"}}</div>
 		<a class="{{if .PageIsSettingsOptions}}active {{end}}item" href="{{.OrgLink}}/settings">

templates/repo/settings/layout_head.tmpl

@@ -1,9 +1,9 @@
 {{template "base/head" .ctxData}}
 <div role="main" aria-label="{{.ctxData.Title}}" class="page-content {{.pageClass}}">
 	{{template "repo/header" .ctxData}}
-	<div class="ui container stackable grid">
+	<div class="ui container flex-container">
 		{{template "repo/settings/navbar" .ctxData}}
-		<div class="twelve wide column">
+		<div class="flex-container-main">
 			{{template "base/alert" .ctxData}}
 			{{/* block: repo-setting-content */}}
 

templates/repo/settings/navbar.tmpl

@@ -1,4 +1,4 @@
-<div class="four wide column">
+<div class="flex-container-nav">
 	<div class="ui fluid vertical menu">
 		<div class="header item">{{ctx.Locale.Tr "repo.settings"}}</div>
 		<a class="{{if .PageIsSettingsOptions}}active {{end}}item" href="{{.RepoLink}}/settings">