V13: Fix members while using basic auth.

[nikolajlauridsen]

Fixes #18186.

Using basic auth breaks member functionality. This turned out to be multiple issues.

1. AuthenticateBackOfficeAsync updated the HttpContext user to the backoffice authenticated one, this breaks members entirely when using basic auth since the backoffice identity will then be the only one ever assigned to the user. I've fixed this by flowing any authenticated identity that is not the backoffice one to the new principal.
2. The Member manager always assumed that the member was the only identity, I've fixed this by now specifically looking for the identity with `IdentityConstants.ApplicationScheme' as the authentication type, I've created an extension for this since it's also used in 3.
3. The login status snippet also assumed that the member identity would be the only one, I've fixed that by using the extension from point 2.

Testing

1. Enable basic auth:

  "Umbraco": {
    "CMS": {
      "BasicAuth": {
        "Enabled": true
      },
{...}

2. Create a member
3. Login to the site
4. Ensure that MemberManager.IsLoggedIn() returns the correct response
5. Ensure that MemberManager.GetCurrentMemberAsync() returns the correct member.
6. Ensure that the Login status work
7. Ensure that backoffice still works.

I used this beauty of a test page to test:

@using Umbraco.Cms.Core.Security
@using Umbraco.Cms.Web.Common.PublishedModels;
@inherits Umbraco.Cms.Web.Common.Views.UmbracoViewPage;
@using ContentModels = Umbraco.Cms.Web.Common.PublishedModels;
@inject IMemberManager MemberManager
@{
	Layout = null;
}

<h1>Member Tester 90000</h1>

<p>Is member logged in: @MemberManager.IsLoggedIn()</p>

@{
    if (MemberManager.IsLoggedIn())
    {
        var currentMember = await MemberManager.GetCurrentMemberAsync();

        if (currentMember is null)
        {
            <p>Current member not found :(</p>
        }
        else
        {
            <p>@currentMember.Name is here...</p>
        }
    }
}


<hr>

<h1>Login</h1>

@await Html.PartialAsync("MyMemberLogin")

<hr>

<h1>Login Status</h1>

@await Html.PartialAsync("MyMemberStatus")

[AndyButland]

Code looks good... will now test it out.

[AndyButland]

And all tests out great - nice work. There was just one tiny comment about a comment to resolve, then it's fine from my side to merge in.