Fix flash message for flex-container (#30657)

(cherry picked from commit dd2aaadce3ecd3134a1ba0c82c5aaa05d6c11b2b)

Conflicts:
	templates/admin/layout_head.tmpl
	    mostly already done by https://codeberg.org/forgejo/forgejo/pulls/3087
	web_src/css/base.css
            the conflict is because
            https://codeberg.org/forgejo/forgejo/pulls/3350
            skipped Remove fomantic menu module (gitea#30325)
            and it was not ported.

templates/admin/layout_head.tmpl

@@ -1,6 +1,6 @@
 {{template "base/head" .ctxData}}
 <div role="main" aria-label="{{.ctxData.Title}}" class="page-content {{.pageClass}}">
-	<div class="ui container flex-container">
+	<div class="ui container fluid padded flex-container">
 		{{template "admin/navbar" .ctxData}}
 		<div class="flex-container-main">
 			{{template "base/alert" .ctxData}}

templates/user/dashboard/dashboard.tmpl

@@ -1,9 +1,9 @@
 {{template "base/head" .}}
 <div role="main" aria-label="{{.Title}}" class="page-content dashboard feeds">
 	{{template "user/dashboard/navbar" .}}
-	{{template "base/alert" .}}
 	<div class="ui container flex-container">
 		<div class="flex-container-main">
+			{{template "base/alert" .}}
 			{{template "user/heatmap" .}}
 			{{template "user/dashboard/feeds" .}}
 		</div>

web_src/css/base.css

@@ -636,12 +636,6 @@ img.ui.avatar,
   background: var(--color-active);
 }
 
-/* add horizontal margin to elements that are outside top-level of .flex-container or .ui.container */
-.page-content > .flash-message {
-  margin-left: var(--page-margin-x);
-  margin-right: var(--page-margin-x);
-}
-
 .ui.form .fields.error .field textarea,
 .ui.form .fields.error .field select,
 .ui.form .fields.error .field input:not([type]),