OpenIddict should only handle /umbraco/ requests

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

Description

OpenIddict performs initial request context processing for every last request to the server - including front-end requests. While this handling is not terribly costly, there really is no reason for it unless a /umbraco/ request is in progress.

Testing this PR

1. First and foremost, the backoffice should work as per usual 😄
2. Enable verbose logging and verify that the OpenIddict request processing is only invoked for /umbraco/ requests.
3. Setup member auth for the Delivery API and verify that this still works.

Particularly the last point is a little tricky to get right. Get in touch with @kjac for a pair testing session 👍

[bergmania]

LGTM and tests out great 💪