This makes for better error messages.

Consult the description directly.

It used to be delegated to clients.

We used to omit the slash, which users found confusing.

This makes it possible to login on small devices.

They're redundant with the client field.

We will fail malicious paths in openDescriptionFile, but it
doesn't harm to be paranoid early.

Thanks to Jeroen van Veen.

It's no longer easy to test group expiration.

Thanks to Jeroen van Veen for suggesting the feature.

Commit b527c875 added the `kind`
parameter to `addToChatbox`, but the change wasn't reflected here.

I'm not seeing any non-keyframe spatial sync frames, so simplify
the code by only switching spatial layers on keyframes.