Make the 404 page navigable

[warpech]

Pasted from Slack thread started by @mtornwall:

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We have a problem with how the Authorization library handles the unauthorized/unauthenticated cases.
If you are unauthorized to access a particular page it sends a 404 without a body, which crashes the session irrecoverably.

So David and I discussed this and we have the following solution in mind:

1. We add a file /sys/error/404.html or similar, providing a default error page.
2. We change the Authorization library so that it sends new Json() { Html = "/sys/error/404.html" } if user is unauthenticated or unauthorized.

In the future we should extend it so that this becomes the default 404 page for all requests.

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The original thread: https://starcounter.slack.com/archives/C7677F70W/p1549988751039200

[warpech]

Depends on: Palindrom/Palindrom#220

[kegor]

Closing because related PRs were merged.