Make it possible to leave lobbies ++

[toberge]

Depends on #661

Closes #670
Closes #671
Closes #662

Stuff done

• Leave lobby when you leave the lobby
• Leave lobby when you disconnect
• Leave lobby when match is over (ideally you'd be back in the lobby again ready for a new match)
• Players visually leave/enter lobbies (PlayerSelectManager jank fixed/worked around)
• Clean up lobby leaving code so you rather have OurNetworkManagerThatIsYetToBeRenamed -> SteamManager
• Disconnects during matches don't break the game (presently the dummy left by the escapee is unkillable, likely due to authority issues)
• Show correct names (Player 1, 2, etc) in lobby
• Fix refresh of lobby list after lobby exit or match end
• Show loading screen when entering training mode
• Fix chamber-filling animation on loading screen (it only played once 💀)
• Fix spawnpoint selection (it used player ID, which can be > 3 now)

Stuff planned but not done in this PR

• Use SetLobbyJoinable(false) to prevent users from joining an ongoing match and so on

[Fueredoriku]

Seems good! I have a few comments, but I'll leave an approve for you to use at your discretion 😁👍