#8 Add boat preference as an enrollment option

[Rick Hall]

When members sign up for a rowing session, allow them
to indicate a boat preference. This enhancement has
quite a few ramifications.

1) Currently members may sign up just for sweep
rowing, and the sweep boats are coach-assigned. Sweep
boats will continue to be coach assigned.

2) Sculls are currently assigned on a first-come,
first-served basis. The site must track specific boats;
seats available within the boat; and allow signups only
if a boat isn't already filled. Easy for singles; but
more complicated for multi-seat boats.

3) The rowing association requires members to be scull
certified before signing up for a scull. Need a new
role (SCULLER). Change existing MEMBER row to SWEEP
(since that is how it is being used).

4) Need a new field or row to indicate members who
only COX.