Add warning message for import + download images

When import isn't async and downloading images is enabled, we warn the user that import can fail.
Also, added missing translations

src/Wallabag/CoreBundle/Resources/translations/messages.da.yml

@@ -367,6 +367,7 @@ import:
     #     how_to: 'Please select your Readability export and click on the below button to upload and import it.'
     worker:
         # enabled: "Import is made asynchronously. Once the import task is started, an external worker will handle jobs one at a time. The current service is:"
+        # download_images_warning: "You enabled downloading images for your articles. Combined with classic import it can take ages to proceed (or maybe failed). We <strong>strongly recommend</strong> to enable asynchronous import to avoid errors."
     # firefox:
     #    page_title: 'Import > Firefox'
     #    description: "This importer will import all your Firefox bookmarks. Just go to your bookmarks (Ctrl+Maj+O), then into \"Import and backup\", choose \"Backup...\". You will obtain a .json file."
@@ -504,3 +505,8 @@ flashes:
         notice:
             # client_created: 'New client created.'
             # client_deleted: 'Client deleted'
+    user:
+        notice:
+            # added: 'User "%username%" added'
+            # updated: 'User "%username%" updated'
+            # deleted: 'User "%username%" deleted'