Round no longer ends suddenly

This PR proposes changing how the end of the round is handled once the current game mode's end conditions have been satisfied, for example, when the vox return back to darkspace. Instead of forcing the game to immediately end, a crew transfer vote is called instead.

Because this PR touches gameticker, I thought it would be important that devs and admins have a chance to provide input and feedback before it is merged.

Also it would be nice to get feedback on how to cleanup the game modes after the mode has ended.
For heist, I'm thinking that everyone on the skipjack should be turned into an observer. The skipjack has returned to the arkship and isn't coming back any time soon. I don't think any of the other game modes need to do cleanup.