"Shout" or What Jeff has been doing :-) or Added validations for gender, ethinicity, and race

Added BE code for the controller, validation, and storage of gender, ethnicity, and race.

Notable things changed:

- The way student's races is selected has changed. There is now a `races` table that gets populated via the seed file. Then, when a race gets assigned to a `student`, an association table (`student_races`) gets populated with the `student`'s `id` and the `race`'s `id`.
- Student race is selected via checkboxes. (@codesliced This will need styling.)

@codesliced Ready for review

(Re shouting: The race thing turned out to be complicated for a Rails newbie such as me. The code is a little ugly for it in my opinion, but it works. `update_attributes` not working great for `race`, so did a workaround so that things get saved.)