Returning campers and invitation discounts

Two smaller improvements, one on the campers table and one on enrollment invitations.

Spot returning campers on the table. Two new columns are available on the campers table, “Returning camper?” and “Sessions (previous years)”, so you can see who is coming back and what they have been to before.

Discounts apply to every session on an invitation. Discounts set through a payment template or custom payment terms now apply to every session on an enrollment invitation, including sessions with “Exclude from discounts” turned on. The exclusion still holds for discount codes and automatic discounts, so it only affects discounts an admin applies directly.