Force focus on the input box after enter key

This only seems to be a problem on FF Nightly (57.0a1 (2017-09-13)),
but it seems harmless for other browsers.

priv/static/main.js

@@ -118,6 +118,7 @@ window.webmu = (function() {
 	    historyPos = history.length;
 	    dirtyHistory = undefined;
 	    inputbox.value = '';
+	    inputbox.focus();
 	    break;
 	case "ArrowUp":
 	    if (inputbox.value.trim() && historyPos == history.length) {