Remove hundreds of unnecessary queries from /alt-styles page!

I think the Rails query cache handled these anyway? But `SwfAsset` has
a `before_save` hook that checks its zone's info, and
`SwfAsset.preload_manifests` saves all the assets, on the assumption
that saving is a no-op when the record didn't change anyway. And it
basically is!

But I figure that, now that I'm realizing hooks exist, simply not
attempting to save unchanged records is probably a better
representation of what we intend to do. So I'm fixing it like that!

Another potential fix would be to preload the zones for these assets,
but I think that confuses the intent too much; the method itself isn't
using the zones, it's just a weird incidental thing that a save hook
happens to use. (Would probably be better to refactor this old save
hook into a different situation altogether, but that's for another
time!)

app/models/swf_asset.rb

@@ -338,7 +338,7 @@ class SwfAsset < ApplicationRecord
     end
 
     SwfAsset.transaction do
-      swf_assets.each(&:save!)
+      swf_assets.select(&:changed?).each(&:save!)
     end
   end