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/Worker/Notifier.php

@@ -143,7 +143,7 @@ class Notifier
 				}
 			}
 
-			$top_level = $target_item['gravity'] == GRAVITY_PARENT;
+			$top_level = $target_item['gravity'] == Item::GRAVITY_PARENT;
 		}
 
 		$owner = User::getOwnerDataById($uid);