Allow batch deletions

[Frewacom]

Currently, you can only delete a single resource at a time. This is inefficient, since you need to make n requests to remove n items. To fix this, we should provide a separate endpoint (or allow two types of data in the validators) that allow us to delete multiple items at once.

This functionality is primarily needed for deletions, but there might be cases where we want to do batch updates as well (so it is a good idea to make this sort of functionality generalized).