Add overage check on groups invite

What does this MR do and why?

Show an extra confirmation modal if adding a group will cause overage for a subscription.

Exclude already paid users from the calculations. Attention: this MR doesn't exclude guests from Ultimate subscription groups, there is an extra issue to address it #353455 (closed)

Related to #348486 (closed)

Screenshots or screen recordings

Screen_Recording_2022-02-23_at_17.25.18

How to set up and validate locally

Enable the invite modal

Feature.enable(:overage_members_modal)

Make sure that you are running GitLab as SaaS and have check_namespace_plan setting enabled

::Gitlab::CurrentSettings.update(check_namespace_plan: true)

1. Create a group, add a premium subscription for one seat
2. Visit this group page, then Group Information -> Members http://localhost:3000/groups/<your-group>/-/group_members.
3. Click the Invite group button.
4. Add a group in a dropdown and click Invite.
5. Check the modal.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.