Mark system predicates that can't be moved in cluster view

[matthewmcneely]

Suggestion.
Should we add a prompt in the UI that "system predicates can't be moved"?
and a ui mark.
Like
"dgraph.type (*)"

and
(*): System predicate - can't be moved.

Originally posted by @raphael-istari in #397 (review)

[matthewmcneely]

@raphael-istari Actually, maybe we should just filter out those predicates. You can't do anything with them....