Clean-up pending-members alert in usage seats

What does this MR do and why?

This work is implementing 2nd problem - 3rd solution of the SaaS User Caps Pending Member Notification & Pa... (&12463 - closed)

This MR is re-adding previously reverted MRs and applying the fix (add the restriction of namespaces with user caps that have a value set display pending members tab) that is mentioned, the code for that fix is in ee/app/helpers/ee/groups_helper.rb.

Each commit correspond to a previously merge MR:

commit	previously merged MR
!142429 (64d3a553)	Add link to pending_members in group/members (!141584 - merged)
!142429 (8777d36a)	Clean-up pending-members alert in usage seats (!141590 - merged)
!142429 (52ae93fd) (this commit includes the extra fix)	Show pending banner to main namespace owners (!142114 - merged)

Related to #431187 (closed) and &12463 (closed).

MR acceptance checklist

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Screenshots or screen recordings

Storage tab

before	after

Members page

parent group	sub group

How to set up and validate locally

1. Don't checkout the branch yet
2. Simulate a SaaS instance
3. Enable saas_user_caps feature flag from rails console. (rails console then Feature.enable(:saas_user_caps))
4. With a group with an Ultimate plan
5. Navigate to Settings -> Permissions, LFS, 2FA -> User Cap
6. Set the user cap limit to 1
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
10. Checkout the branch and refresh the usage quotas > seats page
11. The banner should not appear on the seats page but in the group's members page