Add utility script to find a user by email (UserService.findAllByEmail looks in identities)

Fix https://gitlab.com/gitlab-org/gitter/webapp/issues/1843

• Add UserService.findAllByEmail(also backsUserService.findByEmail) which can find a user by their identity emails.
• Add utility script node script/utils/find-users-by-email.js --email foo@bar.com
• Persist emails for GitHub users when they sign in (so we have something to find)
  • This was normally sent off to Intercom and could be referenced easily (but we are no longer using Intercom), https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1096
  • Currently it is stored in the UserSchema.emails array but we could insert a GitHub identity but not everyone would have a GitHub identity, also see https://gitlab.com/gitlab-org/gitter/webapp/blob/599aa0d3fd638d43eef0f9c0d8758481f592e1f3/server/services/user-service.js#L116
  • This is still a bit problematic for existing users that don't sign in and we can't reference them if they make a support ticket. But we can always look at our Intercom data 😉

Todo

In production, add indexes:

db.users.createIndex({
  emails: 1
}, {
  background: true
})

db.identities.createIndex({
  email: 1
}, {
  background: true
})

---

Closes https://gitlab.com/gitlab-org/gitter/webapp/issues/1843