Replace visibility level options by dropdown

What does this MR do?

As part of including teammate invitations in the onboarding issues experiment (#232823 (closed)), this MR replaces the visibility options with a dropdown.

Design: https://gitlab.com/groups/gitlab-org/-/uploads/a4003debb6388f64676c40663b3afa73/Invite_during_sign_up.png

Screenshots

before	after

These changes are behind a feature flag, hence no changelog.

In order to see these changes locally, enable the onboarding_issues experiment in the console by running:

Feature.enable_percentage_of_time(:onboarding_issues_experiment_percentage, 100)

Sign in and visit: http://localhost:3000/users/sign_up/groups/new