Added an option to turn user Gravatars on or off

* Option can be found in Administration > General, called
  "Use Gravatar user icons"
* Defaulting Gravatars to off
* Added a helper gravatar_for_mail to check the setting before rendering
  the Gravatar.

  #1776


git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@1972 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/views/users/list.rhtml

@@ -29,7 +29,7 @@
   <tbody>
 <% for user in @users -%>
   <tr class="user <%= cycle("odd", "even") %> <%= %w(anon active registered locked)[user.status] %>">
-	<td class="username"><%= gravatar(user.mail.blank? ? "" : user.mail, :size => "24") %><%= link_to h(user.login), :action => 'edit', :id => user %></td>
+	<td class="username"><%= gravatar_for_mail(user.mail.blank? ? "" : user.mail, :size => "24") %><%= link_to h(user.login), :action => 'edit', :id => user %></td>
 	<td class="firstname"><%= h(user.firstname) %></td>
 	<td class="lastname"><%= h(user.lastname) %></td>
 	<td class="email"><%= mail_to(h(user.mail)) %></td>