[FeedItem] Avoid repeated UID hashing after loading from cache (#1148)

This fixes the following issue:

1. bridge sets unique ids for the items (ids get hashed)
2. items go to the cache
3. on next run items get loaded from cache
4. these items have different ids because they were hashed again
5. they show up twice in feed reader

lib/FeedItem.php

@@ -418,6 +418,9 @@ class FeedItem {
 
 		if(!is_string($uid)) {
 			Debug::log('Unique id must be a string!');
+		} elseif (preg_match('/^[a-f0-9]{40}$/', $uid)) {
+			// keep id if it already is a SHA-1 hash
+			$this->uid = $uid;
 		} else {
 			$this->uid = sha1($uid);
 		}