global: fix current_records_rest.default_endpoint_prefixes update

[nharraud]

Problem:
In ext.py we have this code:

A       @blueprint.record_once
        def extend_default_endpoint_prefixes(_):
             ...
B            current_records_rest = app.extensions['invenio-records-rest']
             ...

C       app.register_blueprint(blueprint)

c calls A right away. B depends on invenio-records-rest being already loaded.

Temporary Solution:
We cannot load the blueprint out of ext.py as create_blueprint needs the config.
Thus the simplest fix for now is to use @app.before_first_request.

Problem with this solution:
This however means that one cannot use default_endpoint_prefixes before the first request.