With this change, the platform users who access content via LTI will be
automatically linked to their platform account instead of the new (anonymous)
one. The following conditions need to be met:
* The `LtiConsumer` should be configured to auto-link the users via email.
* The LTI Consumer should share the user's email using the
`lis_person_contact_email_primary` parameter in the LTI Launch POST data.
This also replaces the one-to-one relationship of the `User` and `LtiUser`
with one-to-many. This way, multiple `LtiUser` objects can refer to the same
`edx_user`. With the auto-linking, multiple LTI Consumers can create
independent `LtiUser` objects with the same `edx_user`.
Co-authored-by: Piotr Surowiec <piotr@surowiec.it>
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