Add alert to seats usage if there are any pending members

What does this MR do and why?

It adds an alert in the group's usage seats page, informing the user (admin/owner) about pending members that are awaiting approvals with a button linking to the "pending members" page.

Note: the link currently will point to an empty page (Vue code will be added in upcoming MR), the feature flag is off so merging this won't affect the users.

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

Screenshots or screen recordings

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 -> General -> 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 the previous step)
7. Navigate to Usage Quota from the left sidebar under Settings
8. You should see an alert that you have pending members

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)