Use web-thumbnailer to retrieve thumbnails

* requires PHP 5.6
  * use blazy on linklist since a lot more thumbs are retrieved
  * thumbnails can be disabled
  * thumbs size is now 120x120
  * thumbs are now cropped to fit the expected size

Fixes #345 #425 #487 #543 #588 #590

inc/web-thumbnailer.json

@@ -0,0 +1,9 @@
+{
+  "settings": {
+    "default": {
+      "_comment": "infinite cache",
+      "cache_duration": -1,
+      "timeout": 60
+    }
+  }
+}