Avoid copying data when sending, improve error handling.

Split out deleting of buttons during file transfer into its own
function.

We now distinguish between tokens that specify an empty username
(sub="") and tokens that don't specify sub.  The latter are
considered invalid for now.

Simulcast doesn't seem to work well with screen sharing: only one
layer is sent, which has very low throughput since we send the low
layer first.  Disable simulcast for screen sharing.

Only the first byte of the VP8 header is mandatory, but we
used to reject packets smaller than 4 bytes.  The major part
of the fix is actually in pion/rtp.

addMapping assumes the piddelta is constant, so it doesn't need
the pid.

When using an identity packetmap, we used to update the next seqno
even when packets were not increasing.  Not a big deal, we'd still
recover at the next sequential packet.

Don't disconnect the client for some more errors.

This does not yet support the new 'need-username' error.

The code turned out not to port to FreeBSD.  Disable the check,
and assume that FreeBSD admins read the docs.

Thanks to Amatis-51.

Pion is unfortunately breaking Go 1.16.

It was introduced in Go 1.19.

The strategy of computing the base URL from the request doesn't
necessarily work if we're behind a reverse proxy.  proxyURL
can be set in cases where our guess is incorrect.

Thanks to Dianne Skoll.

We were computing the scheme incorrectly, which caused us
to compute the wrong websocket URL when -insecure is set.
Thanks to Fabien de Montgolfier.

When we fixed the handling of autolocked groups, we introduced
a bug where we spuriously notify clients even when the description
didn't change.

When we moved the chat history to the client goroutine back in 0.6,
we started sending the history whenever the group configuration changed.
Only send it when first joining the group.  Thanks to Rémi Nollet.

We used to autolock groups when the last user left, but not
when creating a group that was not in memory yet.  Ensure that
groups are autolocked in all cases.

Thanks to Michael Ströder.

This just removes a minor optimisation.

This avoids issues with redirects.

Now that we have an indicator in the users list, it is reasonable
to hide audio-ony peers by default.

It's redundant, the data is already in the stream.