Game status transitions (from ready to underway to finished)

Added skeleton handlers for starting, finishing, and resetting a `MatchupGame`.
We rely on the API user to update a `MatchupGame` by setting specific properties,
which in turn trigger a status update via `Workflow`. This way the API user can
be specific about the values entered into specific fields but leaves the
transitions between states up to the API.

To trigger a `MatchupGame`'s transition from `ready` to `underway`, set the
value for `starts_at`.

To trigger a `MatchupGame`'s transition from `underway` to `finished`, set the
value for `ends_at` and set the `winner`'s `id`.