Create a more fleshed out notion of SetUp phases in server layer

[jkomoros]

Related to #752 and #753 .

Currently set up has to be done before the core engine actually starts the game logic. That requires variants to be done in the core logic, and also is pretty inflexible.

Instead, it should be done in a special Phase in the main game logic. This also allows games do to multiple "rounds" easily.

To do this, we have to have machinery for known-semantics of phases that tell the server app, "at this stage, please show this type of machinery". For example, if you're in "player selection" phase, you might show a pop-over dialog with a different renderer. And if you haven't yet seen the core game renderer, e.g. a "lobby", then you might not even pop over to it from the game creation screen yet, showing the dialog over top of hte game list.

This requires the server to know which phases allow player selection, variant selection, etc. Maybe a new method that takes a phase number and returns whether it's in player selection?

[jkomoros]

• Variants should use this machinery (Have game variants be handled entirely in the server layer, not the core library #753)
• Player selection should use this machinery (Allow more complex team selection mechanics #752)
• When players may join should use this machinery (Rationalize notions of Seats, Active players #755)