Add ability to import/export tagging rules

- Add missing translations
- Add some tests
- Add `/api/taggingrule/export` API endpoint
- Add baggy theme
- Add error message when importing tagging rules failed
- Also fix all translations (I think we are good now)

tests/Wallabag/CoreBundle/fixtures/tagging_rules_admin.json

@@ -0,0 +1,4 @@
+[{
+    "rule": "title matches \"football\"",
+    "tags": ["football"]
+}]