Don't attempt to present if we don't have the present permission,
don't display the friendly popup if we've selected presentation.

They now have the same format as chat messages; the only difference
is that they are not stored in the chat history.

Also adds userMessage method to ServerConnection.

Fix usermessages.

Thanks to Antonin Décimo.

Move custom video controls into its own function.  Remove some
DOM traversals, avoid querySelector.  Remove dead code.

Probably useless, but I'm supersticious.

We were inconsistent with stream creation.