Merge branch 'allowteamchange-fix' into 'next'

Allowteamchange fix

Fixes "allowteamchange no" to actually work properly and NOT desync you from the game in most cases. You can now switch TO spectator freely (as intended it turns out), and switching FROM spectator is prohibited. The "Enter Game" menu option in Match/Tag now displays one of those blue window notices just like with "Change Teams" in CTF/Team Match if you cannot change teams right now. Everything else works as you'd expect.

The de-sync issue was originally reported here https://mb.srb2.org/showthread.php?p=789965#85

See merge request !168