Server: add chat command to kick watcher

Problem: watcher with lags can ruined game for other players.

Solution: add special chat commands for table owner to kick that players.

TODO:
* [ ] add command `/watchers kick username` to kick user from watcher game session;

No file or test is named. Start by locating the server-side chat-command handling and watcher game-session logic, then trace how table ownership and watched players are represented. Done means the table owner can use `/watchers kick username` to remove the named user from the watcher session.

Written by the indexing model from the issue text.