Add way to drop data.

[danielmai]

Today in Ratel there's a "DROP ALL" button to drop all the data and schema in Dgraph.

Dgraph also has a way to drop just the data (not the schema). This should be doable in the Ratel UI.

Here's how drop data would be done via curl:

curl -X POST localhost:8080/alter -d '{"drop_op": "DATA"}'

[paulftw]

Fixed in 60d5ca2