Use two indexes instead of one for hashed urls

When using `OR` in a where clause, a composite index can't be used. We should use a `UNION` to take advantages of it.
Instead, create 2 indexes on each hashed urls and make 2 queries to find an url. It'll be faster than the previous solution.

tests/Wallabag/CoreBundle/Controller/EntryControllerTest.php

@@ -265,6 +265,9 @@ class EntryControllerTest extends WallabagCoreTestCase
         $this->assertContains('/view/', $client->getResponse()->getTargetUrl());
     }
 
+    /**
+     * This test will require an internet connection.
+     */
     public function testPostNewOkUrlExistWithRedirection()
     {
         $this->logInAs('admin');