Add users overage check

What does this MR do and why?

Add a check when adding a new user will cause a overage. In this case we will trigger a modal confirmation window (implemented in !79644 (merged)).

This addresses all cases that causes overage on adding new users except guest users for Ultimate subscription. To see the full list of cases check the #348486 (closed)

Step	Status
Introduce a FF and a modal on FE	!79644 (merged)
Add logic to when we show overage modal and calculate the amount of users for overage	👉 you are here

Related to #348486 (closed)

Screenshots or screen recordings

Screen_Recording_2022-02-18_at_18.21.21

How to set up and validate locally

1. Enable the invite modal

   Feature.enable(:overage_members_modal)

2. Create a group, add a premium subscription for one seat
3. Visit this group page, then Group Information -> Members http://localhost:3000/groups/<your-group>/-/group_members.
4. Click the Invite members button.
5. Add a second user (by id or by email) and click Invite.
6. 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. Darft