Added /user to disallowed urls

Googlebot is generating a lot of traffic like:

GET /user?destination=http://data.gov.uk/data/search?dataset-results-sort=/data/search%3Ftags%3Dsafety%26tags%3Dprisons%26sort%3Dmetadata_modified%2Bdesc HTTP/1.1

a. This is a broken url
b. They're hitting the login link and they shouldn't.