Make customizeable (jitsi?) URL for VIDEO_CHAT

[belonesox]

Now jitsi URL for videochat is hardcoded in
frontend/chat/video/video-chat.ts

const VIDEO_CHAT_SERVER = "https://meet.jit.si";
const VIDEO_UPDATE_INTERVAL_MS = 30 * 1000;

Better make this as customizeable parameter (cocalc settings), to use

• Custom (local, on-prem) jitsi installations
• May be other, even non-jitsi servers, that support "URLs for videoroom".