Part 2 - Resolve EE/CE duplication in invite_modal_base

• Part 1 - Move isLoading and invalid state to parents of invite_modal_base
• Part 2 - Resolve EE/CE duplication in invite_modal_base

What does this MR do and why?

This MR addresses #354003 (closed) and #352970 (closed) by resolving the duplication between the EE/CE invite_modal_base.vue which was added in !79644 (merged).

Screenshots or screen recordings

With Feature.disable(:overage_members_modal):

Screencast_2022-03-04_10_51_24

• I accidentally recorded this video on the Project members page, but the effect is the same 😄

With Feature.enable(:overage_members_modal):

Screencast_2022-03-04_10_54_58

How to set up and validate locally

With Feature.disable(:overage_members_modal):

1. Sign in to the local gdk as root
2. Visit a group members page
3. Use the "Invite group" and "Invite members" modal to test loading and the invalid message
   1. You can test the invalid message by inviting a member that is already added to the group
   2. You can throttle the local network to test out the loader

With Feature.enable(:overage_members_modal):

1. Visit a group members page.
2. Click the Invite members button.
3. Add a user and click Invite.
4. The modal should transition nicely to an "Overages" warning message.

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.

Related to #354003 (closed)