Allow moving units into a captured tile immediately after capturing it

[stevegrossi]

As in Risk, you should be able to move any number of units into a tile you just captured from the tile you attacked from immediately after attacking.

• This should not end your turn
• You should have to move at least the number of units you attacked with. So if I attack from a tile with 7 units and capture a neighboring tile, I should have to move between 3 (the number I attacked with) and 6 (the maximum) units into the newly-captured tile.

This poses some novel challenges:

• For the first time, this would introduce a mandatory action. After attacking, the current player would need to specify how many units to move before they could take any other action.
• We'd need to prompt only the current player for this action, so we'd need a way for the GameServer to send a message to only one particular player (other players should not be prompted for how many units to move)

And of course, the AI would need to know how to deal with this.

I think the UI introduced in #26 will be helpful here.

[stevegrossi]

I'm not actually sure about this change. It is consistent with Risk rules, but it's pretty unrealistic that it would allow you to march a massive army entirely around the entire world in a single turn (though that does incentivize defense-in-depth). I kind of like how things work now where you can move at most 3 units into a tile after capturing it, allowing for a bit of momentum after victory but not a massive wrecking ball rolling over entire continents.

Perhaps most realistic would be the ability to move any number of units into a captured tile, but preventing those units from attacking again in the same turn. This is what Compact Conflict does: you basically can't initiate attacks from tiles you didn't control at the start of the turn.

I suppose I could start by implementing this change, and then consider the "no chaining attacks" rule afterward if things need slowing down.