Save teacher events AFTER optiondates have been saved

[bernhard-wunderbyte]

Currently, dates might not be saved for teachers if the optiondates are not already saved.
We need to make sure, that dates are also saved for teachers, even if we newly create an option.
We might need a new MOD_BOOKING_EXECUTION_POSTSAVE_AFTER for this, so this happens AFTER the dates
have been created.