Add a command to delete all images from the 'images' array of each party
Add a helper command to find the maximum person ID

This is useful if by deleting and reimporting data directly in MongoDB
the maximum person ID in the YNMP database gets out of sync: this
command returns the maximum ID, and it's then your reponsibility what to
do about that. (i.e. It doesn't update it in the database for you.)