We now enable typing of sfu.js.

Simplifies the code, makes the files cachable, fixes permission issues.

There is now a data structure that contains the ultimate truth
about the user's preferences.  We no longer grovel through the
UI components.

We now exit with an error code if the webserver couldn't start.

http.FileServer is inflexible, it is better to reimplement it
rather than trying to work around its limitations.

Keeps go vet from complaining.

Move RTP-specific code into its own file.

This requires a proper parser for commands.

Users find it confusing, especially when the browser disables autoplay.

Also export some fields as thread-safe methods.

These are specific to webclients.

It's not obvious that it's useful, especially now that the writer
is split into multiple threads.  Do retain the dynamically computed
delay in case of audio congestion.

Otherwise recording to disk causes a deadlock.