Deny adding a member if it will take namespace over free user cap
What does this MR do and why?
When a namespace qualifies for free user cap limitations as has no more space left for members, deny the member creation and fail with a message about the failure.
What does this MR NOT do?
We don't update the invite modal in the UI(seen in screenshots on !85317 (merged)) to allow inviting users so that it falls through to this logic (currently the UI blocks this) - we'll do that in https://gitlab.com/gitlab-org/gitlab/-/work_items/116212323
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #371917
Edited by Doug Stull