installation: add Invenio for login/auth/access mgmnt

* With the used Invenio packages Flask-Menu is not initialised
  but required by the installed ones (accessing current_menu)
  so we initialise it in REANA Flask extension.

reana_server/config.py

@@ -14,6 +14,7 @@
 
 from invenio_app.config import APP_DEFAULT_SECURE_HEADERS
 from invenio_oauthclient.contrib import cern
+
 # Database
 # ========
 #: Database URI including user and password