Security: Remove system context in favor of origin #36071
Description
In #35667 we move the infrastructure for setting the origin with things like OriginSettingClient and ThreadContext#stashWithOrigin. origin is a very similar concept to the "system context" which you can access with ThreadContext#markAsSystemContext except it allows the security code to be more precise on which permissions to give which actions. That extra little bit of precision feels good to us so we'd like to migrate all uses of the system context to the origin concept and remove the system context all together.