Allow selecting a group to apply license to

What does this MR do?

Part of gitlab-org/growth/team-tasks#60 (closed). Related to !24979 (merged) (Backend)

When an existing user is buying a plan, show existing groups (if any) to apply the license to, in addition to being able to create a new group.

Since this is behind a feature flag, a changelog is not provided.

To see this code in development setup:

• Add return [{id: 'x', code: 'bronze', price_per_year: 48}] at the top of the plan_data method in ee/app/helpers/subscriptions_helper.rb#36, in order to avoid having to configure and run the customers app
• Add group_data: [{ id: 132, name: 'My first group', users: 3 },{ id: 483, name: 'My second group', users: 12 }].to_json to the subscription_data hash in ee/app/helpers/subscriptions_helper.rb#13.
• Sign in and visit http://localhost:3000/-/subscriptions/new?force_experiment=paid_signup_flow
• Ignore the error warning for failing to load the credit card form