Collapse search input on tablets. closes flarum/core#162

While this solution isn't perfect (perhaps a search icon which opens a
dropdown would be better, like Discourse does) - it'll do for now :)

less/lib/App.less

@@ -266,6 +266,18 @@
   }
 }
 
+@media @tablet {
+  .Header-secondary .Search {
+    input {
+      width: 1px;
+      background: transparent;
+    }
+    &:not(.active) input {
+      padding-right: 0;
+    }
+  }
+}
+
 // ------------------------------------
 // Content Area