feature update

- add sidebar breadcrumb
- do not show dot when latest_post_id is not set

src/Sidebar.js

@@ -7,7 +7,7 @@ export function Sidebar(props) {
             <div className="sidebar-shadow" onClick={props.do_close} />
             <div className="sidebar">
                 <p className="sidebar-title">
-                    <a onClick={props.do_close}>&nbsp;×&nbsp;</a>
+                    <a onClick={props.do_close}>&nbsp;<span className="icon icon-back" />&nbsp;</a>
                     {props.title}
                 </p>
                 {props.content}