Generate message.pm_with_url more directly from ids.

We have added people.pm_with_url(message), which computes a
PM url from a private message using user ids rather than emails.

We call this in add_message_metadata(), since the slugs will
be valid even if emails change, so we don't need to compute
them on the fly during message rendering.

static/js/message_store.js

@@ -153,6 +153,7 @@ function add_message_metadata(message) {
         message.reply_to = util.normalize_recipients(
                 exports.get_pm_emails(message));
         message.display_reply_to = exports.get_pm_full_names(message);
+        message.pm_with_url = people.pm_with_url(message);
 
         exports.process_message_for_recent_private_messages(message);
         break;