Also dynamically compute controlbox height (for drag-to-resize)

css/converse.css

@@ -967,6 +967,7 @@ dl.add-converse-contact {
   position: relative;
   margin: 0.5em 0 0 0;
   height: 254px;
+  height: calc(100% - 70px);
 }
 #conversejs dd.available-chatroom {
   overflow-x: hidden;

less/converse.less

@@ -1063,6 +1063,7 @@ dl.add-converse-contact {
     position: relative;
     margin: 0.5em 0 0 0;
     height: 254px;
+    height: ~"calc(100% - 70px)";
 }
 
 #conversejs dd.available-chatroom {