FIX: Cache all flags multisite-safe (#28204)

This fixes an N1 in topics when loading all flags
and also makes the cache multisite-safe, followup
to fb7cc2d3754d23e4c30fa2794eb7ac84ffdc667e

spec/models/flag_spec.rb

@@ -3,6 +3,8 @@
 RSpec.describe Flag, type: :model do
   after(:each) { Flag.reset_flag_settings! }
 
+  use_redis_snapshotting
+
   it "has id lower than 1000 for system flags" do
     flag = Fabricate(:flag, id: 1)
     expect(flag.system?).to be true