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="https://github.com/go-gitea/gitea/assets/115237/1358dab4-55c0-40ce-a4d5-673099304f3d">
<img width="1269" alt="Screenshot 2023-07-21 at 19 35 42"
src="https://github.com/go-gitea/gitea/assets/115237/34812f6d-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">

templates/user/settings/layout_head.tmpl

@@ -1,8 +1,8 @@
 {{template "base/head" .ctxData}}
 <div role="main" aria-label="{{.ctxData.Title}}" class="page-content {{.pageClass}}">
-	<div class="ui container stackable grid">
+	<div class="ui container flex-container">
 		{{template "user/settings/navbar" .ctxData}}
-		<div class="twelve wide column">
+		<div class="flex-container-main">
 			{{template "base/alert" .ctxData}}
 			{{/* block: user-setting-content */}}
 

templates/user/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">{{.locale.Tr "settings"}}</div>
 		<a class="{{if .PageIsSettingsProfile}}active {{end}}item" href="{{AppSubUrl}}/user/settings">

web_src/css/admin.css

@@ -1,26 +1,3 @@
-.admin-container {
-  margin-top: 15px;
-  display: flex !important;
-  gap: 16px;
-}
-
-.admin-nav {
-  width: 240px;
-}
-
-.admin-main {
-  flex: 1;
-}
-
-@media (max-width: 767.98px) {
-  .admin-container {
-    flex-direction: column;
-  }
-  .admin-nav {
-    width: auto;
-  }
-}
-
 .admin.hooks .list > .item:not(:first-child) {
   border-top: 1px solid var(--color-secondary);
   padding: 0.25rem 1rem;

web_src/css/index.css

@@ -11,6 +11,7 @@
 @import "./modules/toast.css";
 @import "./modules/divider.css";
 @import "./modules/svg.css";
+@import "./modules/flexcontainer.css";
 
 @import "./shared/issuelist.css";
 @import "./shared/milestone.css";

web_src/css/modules/flexcontainer.css

@@ -0,0 +1,23 @@
+/* container for full-page content with sidebar on left */
+
+.flex-container {
+  display: flex !important;
+  gap: 16px;
+}
+
+.flex-container-nav {
+  width: 240px;
+}
+
+.flex-container-main {
+  flex: 1;
+}
+
+@media (max-width: 767.98px) {
+  .flex-container {
+    flex-direction: column;
+  }
+  .flex-container-nav {
+    width: auto;
+  }
+}