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

boot.php

@@ -59,18 +59,6 @@ define('CP_USERS_AND_GLOBAL',       2);
  * @}
  */
 
-/**
- * @name Gravity
- *
- * Item weight for query ordering
- * @{
- */
-define('GRAVITY_PARENT',       0);
-define('GRAVITY_ACTIVITY',     3);
-define('GRAVITY_COMMENT',      6);
-define('GRAVITY_UNKNOWN',      9);
-/* @}*/
-
 /**
  * @name Priority
  *