Block user contribution reassignment to admin users

About

This issue came from this discussion https://gitlab.com/gitlab-sirt/shared-incidents/incident_5529/-/issues/62#note_2014390138.

Proposal

Imported user contributions should not be mapped to admin users on .com. These are users who are User.admin? (being verified in #474963 (comment 2021409550)).

We will add a new application setting. The application setting should be added to the Settings > General, Import and export settings section:

Toggle "Allow contributions to be mapped to admin users" - off by default everywhere, on .com and SMs, as it's more secure state.

We will add documentation for:

• The application setting in the admin docs https://docs.gitlab.com/ee/administration/settings/import_and_export_settings.html
• The behaviour of this setting in the user mapping docs