Fix callback order for nested blueprints

Handlers registered via url_value_preprocessor, before_request,
context_processor, and url_defaults are called in downward order: First
on the app and last on the current blueprint.

Handlers registered via after_request and teardown_request are called
in upward order: First on the current blueprint and last on the app.

CHANGES.rst

@@ -21,6 +21,9 @@ Unreleased
     :issue:`4096`
 -   The CLI loader handles ``**kwargs`` in a ``create_app`` function.
     :issue:`4170`
+-   Fix the order of ``before_request`` and other callbacks that trigger
+    before the view returns. They are called from the app down to the
+    closest nested blueprint. :issue:`4229`
 
 
 Version 2.0.1