Add seat assignment hint to invite modal

What does this MR do and why?

Under the seat assignment model, invite search only returns users who already hold a seat in the root namespace (!249867). The invite modal now states this in its intro text and links to Usage Quotas > Seats.

Targets !249867

References

Related to https://gitlab.com/gitlab-org/gitlab/-/work_items/614257+

How to set up and validate locally

1. Start your local SaaS Ultimate GitLab instance
2. Enable the seat_assignment_model feature flag for a group
3. Set seat_assignment_model_enabled: true on that group's namespace_settings
4. Open the group's Members page invite modal — the seat assignment hint should be visible in the intro text, with a link to Usage Quotas > Seats

before	after