✨(backend) manage reconciliation requests for user accounts

[Ash-Crow]

Purpose

Fix #1616.

For now, the reconciliation requests are imported through CSV in the Django admin, which sends confirmation email to both addresses. When both are checked, the actual reconciliation is processed, in a threefold process (update document acess, update invitations, update user status.)

Proposal

• New UserReconciliationCsvImport model to manage the import of reconciliation requests through a task (user_reconciliation_csv_import_job)
• New UserReconciliation model to store the user reconciliation requests themselves (a row = a active_user/inactive_user pair)
  • On save, a confirmation email is sent to the users
• A process_reconciliation admin action process the action on the requested entries, if both emails have been checked.
  • Bulk update the DocumentAccess items, while managing the case where both users have access to the document (keeping the higher role)
  • Bulk update the Invitation items, while managing the case where both users have invitations for the document (keeping the higher role)
  • Bulk update the is_active status on both users
  • Manage the case where an email address already has access and one has an invitation (?)
• Write unit tests