Remember most recent department selection in Flask session

[redshiftzero]

Complementary approach to #366.

Remember the most recent department selection a user has made in the session i.e. on Flask's session object:

from Flask import session

@auth.route('/a-view/')
def my_view_that_involves_a_form():
    ...
    session['default_dept'] = selected_department
    ...

The nice feature with this approach is that this makes for a smoother experience for people that haven't made a user account yet. Check out this link for more on the Flask session object.