Implement "round" concept

[with-heart]

Users play random civs in rounds.

Once all of the available civs have been played (or if the user hasn't played any civs yet), a new round is started.

The round continues until all enabled civs have been played, at which point a new round is started.

If the user enables a civ that has been played since the round started, the civ remains in the played state.

If the user enables a civ that hasn't been played since the round started, the civ remains in the unplayed state.

This allows users to enable/disable civs without losing their "last played" state.