Revert plugin to working state from invenio 12

This reverts commit 640109d77e.

invenio/plugin/custom_invenio_plugin/__init__.py

@@ -1,24 +1,14 @@
 from flask import redirect
-from flask_menu import current_menu
+from flask_menu import register_menu
 
 
 def init_app(app):
     """Initialize application."""
 
     @app.route("/amdmodel")
+    @register_menu(app, ".main.amdmodel", "About AM-D-Model")
     def redirect_to_amdmodel():
         """Redirect to the AM-D-Model website."""
         return redirect("https://am-d-model.eu")
 
-    def register_menu_items():
-        with app.app_context():
-            menu_item = current_menu.submenu(".main.amdmodel")
-            menu_item.register(
-                endpoint='redirect_to_amdmodel',
-                text='About AM-D-Model'
-            )
-
-    with app.app_context():
-        register_menu_items()
-
     return app