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.

Adds a round black semi-transparent background to the white cross icon,
making it visible in any situation.

Instead of buffering the last keyframe, we merely keep track of its
seqno, and use the main cache for recovering.  We also send the whole
sequence of packets rather than just the keyframe itself.

Rewriting is not useful for VP9, and even harmful, as it breaks the
offsets in the scalability structure.

If the client requested a low-resolution stream and there is none,
switch to SID=0.