Make public access endpoints work correctly

[kjac]

Prerequisites

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

Description

There are a few issues with the "get" and "create" endpoints for public access.

Create

1. Upon successful creation of a public access rule, the endpoint yields the wrong "created at" location. The correct location to retrieve the created rule is /document/[document key]/public-access.

GET

1. The HTTP 200 OK response is not documented towards OpenAPI.
2. When queried for a non-existing public access rule, the endpoint should return HTTP 404 NotFound.
3. When queried for a public access rule with one or more explicit members, the operation fails because the umbraco mapper does not know how to map IMember to MemberItemResponseModel.
4. Fixing the previous problem uncovers that public access rules with explicit members yields a MemberItemResponseModel that contains all existing member groups instead of none.

Testing this PR

Use Swagger to ensure that the public access endpoints work as expected.

[Migaroez]

Looks good, works as described

[Migaroez]

Unless I read this wrong this means that

• it is possible for the attempt to succeed without a result
• if it does, we return a 500

That's the goal?

[kjac]

The latter ShouldNotHappen™ but the Result is a nullable value, so -- need a null check for the compiler to stop complaining.

[Migaroez]

ShouldNotHappen™ => 500 sounds good to me. Merging!