User may define system fields on object but cant query them

[AndrewSisley]

For example they can define:

type Author {
	name: String
	books: [Book]
	_count: String
}

but they cannot query _count: String as it is an aggregate system-field. The definition of the collection should fail with a nice error explaining this. _count won't be the only system field where this is an issue, please fix all of them.

TestView_OneToManyWithCount_Errors documents how this can cause problems later in the user-flow, the test should be altered or removed when fixing this issue.