Add "allow bypass of placeholder user confirmation" admin setting

This setting is only aimed towards self-managed users.

• Add an application setting for the feature in the backend. We should consider adding an expiry to the setting as suggested here.
• Add UI to Admin area > Settings > General, Import and export settings section. URL /admin/application_settings/general#js-import-export-settings.
  • Add a new checkbox after Allow contribution mapping to administrators: Allow bypass of placeholder user confirmation. Note Technical Writing review of the copy needed.
  • Add link to documentation on how this works.

Note:

• This change requires adding a new column in the database.
• The new setting should be behind a feature flag importer_user_mapping_allow_bypass_of_confirmation.