MT#16806 show maintenance mode banner

Change-Id: If3dedeb96c8711b1ca7fab3db573e62ec2434c7b

ngcp_panel.conf

@@ -10,6 +10,10 @@ log4perl.appender.Default.layout=PatternLayout
 log4perl.appender.Default.layout.ConversionPattern=%d{ISO8601} [%p] [%F +%L] %m{chomp}%n
 # perhaps also add: host=%H pid=%P
 
+<general>
+    maintenance_mode 0
+</general>
+
 <api_debug_opts>
     allow_fake_client_time 1
     allow_delay_commit 1

share/layout/body.tt

@@ -75,6 +75,11 @@
     </div>
     <div id="content">
         <div class="container">
+            [% IF c.config.general.maintenance_mode -%]
+                <div class="alert alert-error" style="font-size:150%">
+                    <strong>[% c.loc('Maintenance mode is enabled! Please do not perform any write operations until maintenance mode has been switched off!') %]</strong>
+                </div>
+            [% END -%]
             [% content %]
         </div>
     </div>