Merge branch '443554-table-to-support-user-reassignment' into 'master'

Add table and model to enable post-import contribution assignments

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/156241



Merged-by: Michał Zając <mzajac@gitlab.com>
Approved-by: Sam Word <sword@gitlab.com>
Approved-by: Rodrigo Tomonari <rtomonari@gitlab.com>
Approved-by: Brian Williams <bwilliams@gitlab.com>
Approved-by: Michał Zając <mzajac@gitlab.com>
Reviewed-by: Michał Zając <mzajac@gitlab.com>
Reviewed-by: Brian Williams <bwilliams@gitlab.com>
Reviewed-by: Sam Word <sword@gitlab.com>
Co-authored-by: Luke Duncalfe <lduncalfe@eml.cc>