users: add /profile page (fix #4151).

* Add /profile, /profile.json endpoints.
* Make "My Account" link to /profile.
* Add 'User ID' field to profile page.

app/views/users/_statistics.html.erb

@@ -2,6 +2,10 @@
   <h2>Statistics</h2>
   <table width="100%" class="user-statistics">
     <tbody>
+      <tr>
+        <th>User ID</th>
+        <td><%= user.id %></td>
+      </tr>
       <tr>
         <th>Join Date</th>
         <td><%= presenter.join_date %></td>