epub: fix exception when articles have the same title

This commit fixes an exception occuring when exporting as epub several
articles with the same title. The chapter filename is now derived from
title and url.

Fixes #3642

Signed-off-by: Kevin Decherf <kevin@kdecherf.com>

src/Wallabag/CoreBundle/Helper/EntriesExport.php

@@ -188,7 +188,7 @@ class EntriesExport
             foreach ($entry->getTags() as $tag) {
                 $book->setSubject($tag->getLabel());
             }
-            $filename = sha1($entry->getTitle());
+            $filename = sha1(sprintf('%s:%s', $entry->getUrl(), $entry->getTitle()));
 
             $publishedBy = $entry->getPublishedBy();
             $authors = $this->translator->trans('export.unknown');