Adds support for query parameter "hideMenu" (#170)

This adds the ability to load a board with the menu hidden; it provides a form of "read-only" mode, but the key bindings are still active.
Use with, e.g.:
http://localhost:5001/boards/qYAZyGdym6HqjnfaGI4yNtpdx0IMHd9qp6kaJPwbJqE-?hideMenu

The alternative, proposed in #116, was to remove the menu completely, but this would require more substantial changes to board.js (which hangs on "Loading..." if the menu is removed).
Related to #150
Related to #116

client-data/board.html

 
 	<div id="loadingMessage">{{translations.loading}}</div>
 
-	<div id="menu">
+	<div id="menu" {{#hideMenu}}style="display:none;"{{/hideMenu}}>
 		<div id="menuItems">
 			<ul id="tools" class="tools">
 				<li class="tool" tabindex="-1">

server/templating.js

     const boardUriComponent = parts[1];
     params["boardUriComponent"] = boardUriComponent;
     params["board"] = decodeURIComponent(boardUriComponent);
+    params["hideMenu"] = parsedUrl.search.includes("hideMenu");
     return params;
   }
 }