Amend user start node handling

[kjac]

Prerequisites

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

🚨 Please coordinate the merge of this PR with the client 🚨

The matching client PR is here: umbraco/Umbraco.CMS.Backoffice#1727

Description

A long time ago we hardcoded tree root access, because we did not have user permissions properly wired up. We do now 😄 so now it's time to enforce it.

Another thing missing is the ability to define root access at user level in V14, just as we can in V13. It is currently possible at group level in V14, but not yet at user level.

The last piece of this particular puzzle is making sure we don't request the recycle bins if the current user does not have root access. This will yield a 401, which will trigger a "please log in again" dialog --- and 'round and 'round we go. In other words, the "current user" model needs to be enriched with "has root access" to document and media, respectively. The client can then use this to hide the recycle bin when applicable.

Testing this PR

It's probably best to test this in Swagger. The initial setup of users and groups can be done in the V14 UI, though.

1. Users without root access should not be allowed to request the recycle bin endpoints.
2. It should be possible to define root access at user level.
3. The current user's calculated root access should be exposed correctly through the "current user" endpoint.
4. Users with root access should not experience any changes by this PR.

[Migaroez]

I think the integration tests can be improved as there is some multi assert smell present.

[Migaroez]

🥰

[Migaroez]

Bit of a strange message when trying the recycle bin root endpoint

  "type": "Error",
  "title": "Unauthorized user",
  "status": 401,
  "detail": "The current backoffice user should have access to the tree root"
}

The current backoffice user does not have access to the tree root seems a better reason for why its a 401

[Migaroez]

Some minor remarks about the code.
Tests out as described.