views: use .get() for THEME_FRONTPAGE to avoid hard dependency

* Switches from direct app.config["THEME_FRONTPAGE"] access to
  app.config.get("THEME_FRONTPAGE") to prevent KeyErrors in
  environments where the config is not set (e.g. alembic upgrades,
  or test contexts)

Author: jrcastro2

invenio_theme/views.py

@@ -22,7 +22,7 @@ def create_blueprint(app):
         static_folder="static",
     )
 
-    if app.config["THEME_FRONTPAGE"]:
+    if app.config.get("THEME_FRONTPAGE"):
         blueprint.add_url_rule("/", "index", view_func=index)
 
     return blueprint