Accept future join dates: remove past-only validation and update tests

lib/membership/member.ex

@@ -473,11 +473,6 @@ defmodule Mv.Membership.Member do
       end
     end
 
-    # Join date not in future
-    validate compare(:join_date, less_than_or_equal_to: &Date.utc_today/0),
-      where: [present(:join_date)],
-      message: "cannot be in the future"
-
     # Exit date not before join date
     validate compare(:exit_date, greater_than: :join_date),
       where: [present([:join_date, :exit_date])],