[DirectMessage] Major plugin rework

This commit does the necessary rework to store private messages
as Notices and to support Federation. The plugin's README presents
some more detail about the changes and future work that is still
required to do.

plugins/DirectMessage/lib/inboxmessagelistitem.php

@@ -1,7 +1,39 @@
 <?php
+// This file is part of GNU social - https://www.gnu.org/software/social
+//
+// GNU social is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// GNU social is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with GNU social.  If not, see <http://www.gnu.org/licenses/>.
 
-if (!defined('GNUSOCIAL')) { exit(1); }
+/**
+ * GNUsocial implementation of Direct Messages
+ *
+ * @package   GNUsocial
+ * @author    Mikael Nordfeldth <mmn@hethane.se>
+ * @author    Bruno Casteleiro <brunoccast@fc.up.pt>
+ * @copyright 2019 Free Software Foundation, Inc http://www.fsf.org
+ * @license   https://www.gnu.org/licenses/agpl.html GNU AGPL v3 or later
+ */
 
+defined('GNUSOCIAL') || die();
+
+/**
+ * Inbox MessageListItem widget
+ *
+ * @category  Plugin
+ * @package   GNUsocial
+ * @author    Evan Prodromou <evan@status.net>
+ * @license   https://www.gnu.org/licenses/agpl.html GNU AGPL v3 or later
+ */
 class InboxMessageListItem extends MessageListItem
 {
     /**
@@ -9,8 +41,8 @@ class InboxMessageListItem extends MessageListItem
      *
      * @return Profile The profile that matches the message
      */
-    function getMessageProfile()
+    function getMessageProfile(): ?Profile
     {
-        return $this->message->getFrom();
+        return $this->message->getProfile();
     }
 }