Rewrite:

- moved constants GRAVITY_* from boot.php to Friendica\Model\Item
- also rewrote some array initialization:

From:
````
<?php
$arr = [];
$arr['foo'] = "FOO";
````

To:
````
<?php
$arr['foo'] = "FOO";
````
- added a few type-hints

src/Module/Update/Network.php

@@ -23,6 +23,7 @@ namespace Friendica\Module\Update;
 
 use Friendica\Core\System;
 use Friendica\DI;
+use Friendica\Model\Item;
 use Friendica\Model\Post;
 use Friendica\Module\Conversation\Network as NetworkModule;
 
@@ -55,7 +56,7 @@ class Network extends NetworkModule
 			} elseif (self::$order === 'received') {
 				// Only load new toplevel posts
 				$conditionFields['unseen'] = true;
-				$conditionFields['gravity'] = GRAVITY_PARENT;
+				$conditionFields['gravity'] = Item::GRAVITY_PARENT;
 			} else {
 				// Load all unseen items
 				$conditionFields['unseen'] = true;