FIX: Cache Discourse.system_user separately for each multisite tenant (#8276)

This was not causing any known issue, because the system user ID is always the same across all sites. However, we should cache this on a per-site basis to be safe.

spec/components/discourse_spec.rb

@@ -130,6 +130,12 @@ describe Discourse do
 
   end
 
+  context '#system_user' do
+    it 'returns the system user' do
+      expect(Discourse.system_user.id).to eq(-1)
+    end
+  end
+
   context "#store" do
 
     it "returns LocalStore by default" do