fix: translation only applied on app start

* the problem was that with gettext the translations for Invenio string
  have been only be calculated on boot time.

* using lazy_gettext instead fixes that, because it calculates
  the translation at the request level

Co-authored-by: Mirek Simek <[email protected]>

invenio_theme/config.py

@@ -9,7 +9,7 @@
 
 """Configuration for Invenio-Theme."""
 
-from invenio_i18n import gettext as _
+from invenio_i18n import lazy_gettext as _
 
 BASE_TEMPLATE = "invenio_theme/page.html"
 """Base template for user facing pages.