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/Core/Protocol.php

@@ -22,6 +22,7 @@
 namespace Friendica\Core;
 
 use Friendica\Database\DBA;
+use Friendica\Model\Item;
 use Friendica\Model\User;
 use Friendica\Network\HTTPException;
 use Friendica\Protocol\Activity;
@@ -139,7 +140,7 @@ class Protocol
 			// create a follow slap
 			$item = [
 				'verb'    => Activity::FOLLOW,
-				'gravity' => GRAVITY_ACTIVITY,
+				'gravity' => Item::GRAVITY_ACTIVITY,
 				'follow'  => $contact['url'],
 				'body'    => '',
 				'title'   => '',
@@ -191,14 +192,16 @@ class Protocol
 
 		if (in_array($protocol, [Protocol::OSTATUS, Protocol::DFRN])) {
 			// create an unfollow slap
-			$item = [];
-			$item['verb'] = Activity::O_UNFOLLOW;
-			$item['gravity'] = GRAVITY_ACTIVITY;
-			$item['follow'] = $contact['url'];
-			$item['body'] = '';
-			$item['title'] = '';
-			$item['guid'] = '';
-			$item['uri-id'] = 0;
+			$item = [
+				'verb'    => Activity::O_UNFOLLOW,
+				'gravity' => Item::GRAVITY_ACTIVITY,
+				'follow'  => $contact['url'],
+				'body'    => '',
+				'title'   => '',
+				'guid'    => '',
+				'uri-id'  => 0,
+			];
+
 			$slap = OStatus::salmon($item, $user);
 
 			if (empty($contact['notify'])) {