Add guest overage modal

What does this MR do and why?

Adds a confirmation modal before users upgrade Guest users to a higher role. Right now this modal appears every time a guest user's access level is increased. This logic will be refined further in #199893 (closed).
Also this is iteration 1 of #363498 (closed) and the values in the modal content (like seats and group name) are dummy values which will be wired with the backend values in the follow up MR - !106515 (closed).
This change is currently behind the feature flag show_overage_on_role_promotion.

Screenshots or screen recordings

How to set up and validate locally

• Open up the rails console rails c and enable the show_overage_on_role_promotion flag (Feature.enable(:show_overage_on_role_promotion))
• Navigate to the project members page for a project (ex: gitlab-org/gitlab-shell/-/project_members)
• Change the Max role value, the modal should appear
  • Clicking off of the modal or clicking "Back" should close the modal
  • Clicking "Continue" will also close the modal (will be updated in a follow up)
• Clicking the "Learn more" link should open a help page in a new tab
• Open the dropdown and re-select the current Max role value. The modal should not appear

---

• Disable the feature flag (Feature.disable(:show_overage_on_role_promotion))
• Ensure that the modal does not appear when changing the Max role

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 #363498 (closed)