Allow users to remove categories, locations and images from models

[Frewacom]

Currently, you can only set and update these values. You can not remove the value completely. For example, if I assign an event to a category, there is no way for me to remove the category completely from the event (other than selecting another category). The same goes for setting related locations and cover images.

Fixing this should be relatively easy. Simply allow the user to specify null (or something like that) to each respective field in the update validators.