Offer first_time_ta field not set

[siefkenj]

The first_time_ta field is never set when a new offer is created. It should probably be set with the following algorithm:

If there is a previous assignment for the TA in the database for a session other than the session whose start date is prior to the current session, set first_time_ta=false. If there is an application for the current session/applicant with either previous_department_ta or previous_uoft_ta set as true first_time_ta=false. Otherwise, first_time_ta=true.

This should probably be done in the active_offers_controller.rb right after the offer is created. (We don't want to put this code in the populate_offer function in the offers model, because it would be strange to have extra database queries in a model.