Fix form action template substitutions on admin pages (backport #11519) (#11531)

* Fix form action template substitution on admin dashboard

* Fix form action template substitution on admin monitoring page

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lauris BH <lauris@nix.lv>

templates/admin/monitor.tmpl

@@ -7,7 +7,7 @@
 			{{.i18n.Tr "admin.monitor.cron"}}
 		</h4>
 		<div class="ui attached table segment">
-			<form method="post" action="{{.AppSubUrl}}/admin">
+			<form method="post" action="{{AppSubUrl}}/admin">
 				{{.CsrfTokenHtml}}
 				<table class="ui very basic striped table">
 					<thead>