Only allow US states and DC for states

Instead of allowing US territories, which _do_ issue license plates
valid in the continental US (see: 'United States license plate designs
and serial formats' on Wikipedia), just stick with 50 states and DC.
This makes for an easier interface. In the future, we should probably
allow an "international" option for any Canadian/Mexican/etc. cars).

Additionally, just use simple dropdown instead of Select2 (as the
element wasn't styled, and it's not terribly hard to find your state in
a list of 50, even on mobile devices).
Properly handle unicode strings

While model fields are properly saved/retrieved as unicode strings,
various signals that try to print a bytestring with formatted unicode
will cause errors. These print statements shouldn't be in production
code anyway, so remove them. Use unicode literals on any module that
deals with input/output of database values.
Improved validation on forms

Establish sensible ranges for integers, give messages for invalid input.
Connect lottery driver status and WIMP car info

Create a JavaScript-aided interface for specifying whether you own, will
drive, or don't have a car for each week's trips.

There was an illogical divide between the participant information
collected and weekly driver status. To early reviewers of the system, it
wasn't terribly clear why one had to specify car ownership again and why
there was no link to car information from the lottery form.

The new system stores information about the most recent car an
individual used, but allows them to select their driver status
week-to-week. This will hopefully increase the reliability of
identifying information about cars (people who drive different cars will
have to complete the form each week, and renters are encouraged to keep
the information up to date).
Style errorlists/errors with red box

Also include missing non-field errors on the leader form (there
shouldn't ever be any, but it's good practice).
Remove Student as an affiliation

Students were mistakenly being treated as MIT affiliates in the code.
Per MITOC rules, students from other institutions get discounted
membership, but shouldn't be given preference in Winter School.

Affiliation could now easily be converted to a boolean, but we may wish
to change the field later, so leave it as-is.