BE [Group owner bypass] Add "allow bypass of placeholder user confirmation" group setting

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 a group setting for the feature in the backend.

The frontend implementation covered by #544024 (closed)

Subsequent work:

The follow up issue #542284 (closed)

• Considerations for an expiry date tied to the feature as suggested here
• implement license/enterprise user checks. Proposals are discussed here and here
• implementing the bypass of user confirmation as a result of allow_bypass_placeholder_confirmation_for_enterprise being true, in accordance with expiration date

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.

A FE MR has been raised but is blocked by the completion of this work: FE [Group owner bypass]: Add form elements to g... (!189546 - merged)