Implement selectNextPoet
Enforce turns: only show line submission when it is the poet's turn.

Order the poet list when the turn changes.
Change the line object so it has a poet object, and the poet object has a
position. This simplifies the way colors are assigned to poets, but the
Poem domain object is now responsible for modifying lines and poets.

A problem remains: when the server is restarted, the poet order is lost
when the poets are re-loaded from the database.