Show pending members in Seats Usage -> Pending Members

What does this MR do and why?

It adds the frontend of the "pending members" page, which is behind saas_user_caps feature flag.

The page lists all pending users or pending invited members for group admins/owners to see. (later they'll be able to approve).

Merge request	Status
Add pending_members route	✅ merged
Add alert to seats usage if there are any pending members	✅ merged
Show pending members in Seats Usage -> Pending Members	👈 You're here

Screenshots

before	after
N/A

I've adjusted the items per page to take the screenshot and present the pagination, but the code is using the default items per page 20.

How to set up and validate locally

1. Enable saas_user_caps feature flag from rails console. (rails console then Feature.enable(:saas_user_caps))
2. With a group with an Ultimate plan
3. Navigate to Settings -> Permissions, LFS, 2FA -> User Cap
4. Set the user cap limit to 1
5. Navigate to the group members
6. Add one or more members (until you are above the user cap limit set in previous step)
7. Navigate to Usage Quota from the left sidebar under Settings
8. You should see an alert that you have pending members
9. Clicking the button in the alert should navigate to a pending members page

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 #333064 (closed)