layout: Apply `overflow:auto` also to `.container` elements

Otherwise content not wrapped with `.content` lets overflow
the entire page.

public/css/icinga/layout.less

@@ -98,6 +98,7 @@
 
   & > .container {
     width: 0;
+    overflow: auto;
     flex: 1 1 auto;
     display: flex;
     flex-direction: column;