Nested blueprints

This allows blueprints to be nested within blueprints via a new
Blueprint.register_blueprint method. This should provide a use case
that has been desired for the past ~10 years.

This works by setting the endpoint name to be the blueprint names,
from parent to child delimeted by "." and then iterating over the
blueprint names in reverse order in the app (from most specific to
most general). This means that the expectation of nesting a blueprint
within a nested blueprint is met.

CHANGES.rst

@@ -69,6 +69,7 @@ Unreleased
     ``@app.route("/login", methods=["POST"])``. :pr:`3907`
 -   Support async views, error handlers, before and after request, and
     teardown functions. :pr:`3412`
+-   Support nesting blueprints. :issue:`593, 1548`, :pr:`3923`
 
 
 Version 1.1.2