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

update.php

@@ -299,7 +299,7 @@ function update_1349()
 	}
 
 	if (!DBA::e("UPDATE `item` INNER JOIN `item-activity` ON `item`.`uri-id` = `item-activity`.`uri-id`
-		SET `vid` = `item-activity`.`activity` + 1 WHERE `gravity` = ? AND (`vid` IS NULL OR `vid` = 0)", GRAVITY_ACTIVITY)) {
+		SET `vid` = `item-activity`.`activity` + 1 WHERE `gravity` = ? AND (`vid` IS NULL OR `vid` = 0)", Item::GRAVITY_ACTIVITY)) {
 		return Update::FAILED;
 	}