RateSvc duplicates starring for the same user

[prydonius]

In https://github.com/kubeapps/ratesvc/blob/master/handler.go#L117 we aren't checking if the current uid is already in the list of StargazerIDs for item fetched from database. This means multiple requests to the PUT endpoint from the same user token with {has_starred: true} can create duplicate entries of the uid in the list.

If the uid is already in the list, we should no-op.

See also vmware-archive/hub#7