Bug 1021218: merge-users.pl fails if both users have bug_user_last_visit entries for the same bug
Bug 1017648: migrate mentors from the whiteboard to the mentor field
Bug 1026421 - "No QA contact" represented by "" instead of null
Bug 1026415 - Some empty fields are present as "" rather than being absent
Bug 1026862: updating a bug via an api or bulk bug update clears the mentor field
Bug 1026400: id, attachment.id, attachment.bug_id are returned as strings, not integers
Merge branch 'master' into production