Replace Flask-BabelEx with Invenio-i18n

* also remove Flask dependency, because that's already covered

tests/conftest.py

@@ -16,7 +16,6 @@ import shutil
 
 import pytest
 from flask import Flask
-from flask_babelex import Babel
 from flask_limiter import Limiter
 from flask_webpackext.manifest import (
     JinjaManifest,
@@ -37,7 +36,7 @@ from invenio_db import InvenioDB, db
 from invenio_files_rest import InvenioFilesREST
 from invenio_files_rest.models import Location
 from invenio_formatter import InvenioFormatter
-from invenio_i18n import InvenioI18N
+from invenio_i18n import Babel, InvenioI18N
 from invenio_indexer import InvenioIndexer
 from invenio_jsonschemas import InvenioJSONSchemas
 from invenio_pidstore import InvenioPIDStore