✨(backend) Add LTI user to Marsha user association

[qbey]

Purpose

To provide a smooth experience for user between LMS use and Marsha's own site use we need to be able to map users from both contexts.

Proposal

This introduces a new database table to store LMS user / Marsha user link.

• [x] Add the LMS/Marsha user correlation table
• [x] Manage the LMS/Marsha user correlation creation