Before creating a UserRoleBinding for a specific org_id, check whether
the org's OrganizationProfile.license_user_limit has been reached. New
users attempting to join a full org receive HTTP 403 with a descriptive
message. Re-assigning a user already in the org (role change) is
unaffected, as is assignment to an org with no OrganizationProfile.
Co-Authored-By: Paperclip <noreply@paperclip.ing>
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