Generate usernames migration

https://github.com/code-dot-org/code-dot-org/pull/204

+ a migration that generates usernames for existing users who don't have them. This is a separate pull request because it will take a while and I'm not sure it will succeed -- the plan is:

1. merge 204
2. run the migration by hand on prod at some point after 204 DTPs
3. merge this PR