Automated assignments

[m-j-mcdonald]

This adds a feature to automatically generate a CSV of assignments. It uses a version of stable marriage to first match registrations to committees and then registrations to countries within those committees. Priority for the suitors is based of registration's stated preferences and priority for the accepters is based off the time of registration. This does not add the assignments to the database; it only creates the CSV (which can then be uploaded into the database). It will not overwrite existing assignments. One improvement that should probably be added is to better match registrations to countries they have already been paired with in other committees, in order to let schools have the same country across as many committees as possible.