css/layout-structure: fix broken columns

There is a border right now, changed box model to avoid influence on
container width

public/css/icinga/layout-structure.less

@@ -94,6 +94,9 @@ html {
 
 /* tmp */
 #main > .container {
+  box-sizing: border-box;
+  -moz-box-sizing: border-box;
+  -webkit-box-sizing: border-box;
   width: 100%;
   height: 100%;
   overflow: auto;
@@ -101,7 +104,7 @@ html {
 }
 
 #layout.twocols #main > .container {
-  width: 49%;
+  width: 50%;
 }