ratel: visually delete nodes

[pjebs]

It would be great if we can delete a node in ratel by clicking on a button.
It is inconvenient to delete using code sometimes. Sometimes during development it's just easier for me to drop the db and start afresh.

An improvement would be to do various common actions visually and see the query generated via some log in ratel as a FYI.

[MichelDiz]

@pjebs Any Ratel issue can be made in https://github.com/dgraph-io/ratel/issues
This issue will be moved to there.

[paulftw]

We've merged in a feature that generates "drop node" query. Interactive dropping the node will be added as a part of a bigger interactive editing feature