When we check for duplicate emails, it's entirely possible that there
are valid instances of this (e.g. a user has been repeatedly created and
deleted, and finally created anew).

This changeset introduces the timestamp data provided by GSuite
(specifically creation and deletion date), and applies the following
heuristics:
- if a user was deleted before another one with the same Email address
  was created, then it can be ignored, and no duplicate email error need
  be raised
- if multiple users with the same email address were all deleted, we can
  safely synchronize that deletion.
- if a user has been deleted after the creation of a user with a
  conflicting email address, we still raise a duplicate email error.