Ensure orphan tag are remove in API

When the association between a tag and an entry is removed, if the tag doesn’t have other entries, we can remove it.

Also add more tests for that part and ensure TagControllerTest is isolated from the rest of the test suite (finally!)

src/Wallabag/ApiBundle/Controller/WallabagRestController.php

@@ -400,6 +400,8 @@ class WallabagRestController extends FOSRestController
             ->getRepository('WallabagCoreBundle:Entry')
             ->removeTag($this->getUser()->getId(), $tag);
 
+        $this->cleanOrphanTag($tag);
+
         $json = $this->get('serializer')->serialize($tag, 'json');
 
         return (new JsonResponse())->setJson($json);
@@ -440,6 +442,8 @@ class WallabagRestController extends FOSRestController
             ->getRepository('WallabagCoreBundle:Entry')
             ->removeTags($this->getUser()->getId(), $tags);
 
+        $this->cleanOrphanTag($tags);
+
         $json = $this->get('serializer')->serialize($tags, 'json');
 
         return (new JsonResponse())->setJson($json);
@@ -464,6 +468,8 @@ class WallabagRestController extends FOSRestController
             ->getRepository('WallabagCoreBundle:Entry')
             ->removeTag($this->getUser()->getId(), $tag);
 
+        $this->cleanOrphanTag($tag);
+
         $json = $this->get('serializer')->serialize($tag, 'json');
 
         return (new JsonResponse())->setJson($json);
@@ -485,6 +491,28 @@ class WallabagRestController extends FOSRestController
         return (new JsonResponse())->setJson($json);
     }
 
+    /**
+     * Remove orphan tag in case no entries are associated to it.
+     *
+     * @param Tag|array $tags
+     */
+    private function cleanOrphanTag($tags)
+    {
+        if (!is_array($tags)) {
+            $tags = [$tags];
+        }
+
+        $em = $this->getDoctrine()->getManager();
+
+        foreach ($tags as $tag) {
+            if (count($tag->getEntries()) === 0) {
+                $em->remove($tag);
+            }
+        }
+
+        $em->flush();
+    }
+
     /**
      * Validate that the first id is equal to the second one.
      * If not, throw exception. It means a user try to access information from an other user.