FE [Group Owner bypass] "allow bypass of placeholder user confirmation" group setting

This frontend issue will merge after the backend #534329 (closed).

This setting is only aimed towards groups with enterprise users. This action should be available only for the Owner of a top-level group, with Premium or Ultimate license.

• Add UI to Group > Settings > General, Permissions and group features section. URL /groups/gitlab-org/-/edit#js-permissions-settings.
  • Add a new checkbox after Membership: Allow bypass of placeholder user confirmation. Note Technical Writing review of the copy needed.
  • Add link to documentation on how this works.
• The new setting should be behind a feature flag group_owner_placeholder_confirmation_bypass importer_user_mapping_allow_bypass_of_confirmation.

For the frontend:

The checkbox should be shown for all top-level groups on GitLab.com, but in disabled state on Free tier (unable to click the box). On Premium and Ultimate in enabled state (i.e. user can change the setting).

• Only show the settings for top-level groups AND when GitLab.com (Group#root? plus Gitlab.com? or Gitlab::SaaS.feature_enabled? check)
• Only enable the setting if the top-level group can configure a verified domain (#domain_verification_available? check which checks their license tier, that they're a top-level group, and that they're on GitLab.com).

Summary of feature (once complete)

We only allow the enterprise bypass when all of these criteria are met:

• Feature flag is enabled AND Group has the setting enabled AND has correct license tier AND user is one of their enterprise users AND it's GitLab.com AND group is top-level group.