Integrate authorization into Blazor router

[SteveSandersonMS]

Tests are in progress, but this does work. Update: tests are done and this is ready. This builds on my previous PR, #10487

Posting the PR a bit early in case anyone sounds any alarms about the way it determines what authorization attributes to apply (I've done it in a very simple way, but it's hard to compare against the giant metadata system used for controllers/pages).

[SteveSandersonMS]

@rynowak @danroth27

On a design point, I want to emphasise that when authorization fails, although we don't display the page we do still use the layouts nominated by that page. This is because what other layout would we use? We can either:

• Decide this is sensible and carry on
• Decide we think the page's layout should not be used because it might contain sensitive content, and instead have a DefaultLayout on the Router, and use that instead. We also have a need for a DefaultLayout to fix Router displays NotFoundContent without any layout #10445, so maybe it's the right way forwards.

I'd lean towards the second option, but not blocking the merging of this PR for it.

[CornedBee]

This moved PageLayout out of the Layouts namespace, but failed to update the file location or the ref assembly.

[danroth27]

@CornedBee Thanks for letting us know! Could you please open an issue for this so that we can track it?

[CornedBee]

@danroth27 #11240 is essentially this, except the reporter apparently found a workaround for his use case and closed the bug. Maybe you can reopen it? If not, I can create a new one.

[danroth27]

@CornedBee Thanks for bringing this to our attention! I've reopened #11240 so that we can address this.