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/Api/Mastodon/Statuses/Bookmark.php

@@ -47,7 +47,7 @@ class Bookmark extends BaseApi
 			DI::mstdnError()->RecordNotFound();
 		}
 
-		if ($item['gravity'] != GRAVITY_PARENT) {
+		if ($item['gravity'] != Item::GRAVITY_PARENT) {
 			DI::mstdnError()->UnprocessableEntity(DI::l10n()->t('Only starting posts can be bookmarked'));
 		}