Add deleted_at IS NULL check for GetUserIDsByExternalAccounts
Created by: pjlast
This PR addresses https://sourcegraph.atlassian.net/browse/CLOUD-309 where a deleted user external account is still used to associate GitHub userIDs to Sourcegraph User IDs. As stated in the ticket description, we don't know of a way in which this can be abused, but it does lead to an inconsistent user experience.
Test plan
Added unit test and tested the behaviour manually