users: disable upgrading to platinum.

Disable the ability to upgrade to Platinum. The Platinum level still
exists, but users can no longer upgrade to it. Sales of Platinum are
being disabled in preparation of increasing the tag limit for Gold
users.

app/views/users/_secondary_links.html.erb

@@ -13,12 +13,12 @@
       <%= subnav_link_to "Settings", settings_path %>
       <%= subnav_link_to "Messages #{unread_dmail_indicator(CurrentUser.user)}", dmails_path(search: { folder: "received" }) %>
 
-      <% if !@user.is_platinum? %>
+      <% if !@user.is_gold? %>
         <%= subnav_link_to "Upgrade", new_user_upgrade_path %>
       <% end %>
     <% else %>
       <%= subnav_link_to "Send message", new_dmail_path(:dmail => {:to_id => @user.id}) %>
-      <% if !@user.is_platinum? %>
+      <% if !@user.is_gold? %>
         <%= subnav_link_to "Gift upgrade", new_user_upgrade_path(:user_id => @user.id) %>
       <% end %>
     <% end %>

app/views/users/_statistics.html.erb

@@ -84,7 +84,7 @@
         <td>
           <%= user.level_string %>
 
-          <% if !user.is_platinum? %>
+          <% if !user.is_gold? %>
             <% if CurrentUser.user == user %>
               (<%= link_to "Upgrade account", new_user_upgrade_path %>)
             <% else %>

app/views/users/show.html+tooltip.erb

@@ -52,7 +52,7 @@
         <% end %>
       <% end %>
 
-      <% if !@user.is_platinum? %>
+      <% if !@user.is_gold? %>
         <% menu.item do %>
           <%= link_to new_user_upgrade_path(user_id: @user.id) do %>
             <%= gift_icon %> Gift Upgrade