[CORE] Add ActivityPub plugin

This is not the same as the one in https://notabug.org/diogo/gnu-social-activitypub-plugin
Differences to the first "release"
-> Doesn't use guzzle nor has any composer dependencies
-> Supports HTTP Signatures
-> Has basic l10n/i18n
-> Some minor bug fixes

plugins/ActivityPub/classes/Activitypub_follow.php

@@ -0,0 +1,91 @@
+<?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/>.
+
+/**
+ * ActivityPub implementation for GNU social
+ *
+ * @package   GNUsocial
+ * @author    Diogo Cordeiro <diogo@fc.up.pt>
+ * @copyright 2018-2019 Free Software Foundation, Inc http://www.fsf.org
+ * @license   https://www.gnu.org/licenses/agpl.html GNU AGPL v3 or later
+ * @link      http://www.gnu.org/software/social/
+ */
+
+defined('GNUSOCIAL') || die();
+
+/**
+ * ActivityPub error representation
+ *
+ * @category  Plugin
+ * @package   GNUsocial
+ * @author    Diogo Cordeiro <diogo@fc.up.pt>
+ * @license   https://www.gnu.org/licenses/agpl.html GNU AGPL v3 or later
+ */
+class Activitypub_follow extends Managed_DataObject
+{
+    /**
+     * Generates an ActivityPub representation of a subscription
+     *
+     * @author Diogo Cordeiro <diogo@fc.up.pt>
+     * @param string $actor
+     * @param string $object
+     * @return array pretty array to be used in a response
+     */
+    public static function follow_to_array($actor, $object)
+    {
+        $res = [
+            '@context' => 'https://www.w3.org/ns/activitystreams',
+            'id'     => common_root_url().'follow_from_'.urlencode($actor).'_to_'.urlencode($object),
+            'type'   => 'Follow',
+            'actor'  => $actor,
+            'object' => $object
+       ];
+        return $res;
+    }
+
+    /**
+     * Handles a Follow Activity received by our inbox.
+     *
+     * @param Profile $actor_profile Remote Actor
+     * @param string $object Local Actor
+     * @throws AlreadyFulfilledException
+     * @throws HTTP_Request2_Exception
+     * @throws NoProfileException
+     * @throws ServerException
+     * @author Diogo Cordeiro <diogo@fc.up.pt>
+     */
+    public static function follow($actor_profile, $object)
+    {
+        // Get Actor's Aprofile
+        $actor_aprofile = Activitypub_profile::from_profile($actor_profile);
+
+        // Get Object profile
+        $object_profile = new Activitypub_explorer;
+        $object_profile = $object_profile->lookup($object)[0];
+
+        if (!Subscription::exists($actor_profile, $object_profile)) {
+            Subscription::start($actor_profile, $object_profile);
+            common_debug('ActivityPubPlugin: Accepted Follow request from '.ActivityPubPlugin::actor_uri($actor_profile).' to '.$object);
+        } else {
+            common_debug('ActivityPubPlugin: Received a repeated Follow request from '.ActivityPubPlugin::actor_uri($actor_profile).' to '.$object);
+        }
+
+        // Notify remote instance that we have accepted their request
+        common_debug('ActivityPubPlugin: Notifying remote instance that we have accepted their Follow request request from '.ActivityPubPlugin::actor_uri($actor_profile).' to '.$object);
+        $postman = new Activitypub_postman($actor_profile, [$actor_aprofile]);
+        $postman->accept_follow();
+    }
+}