R1:5664c838fbbc

Reduce thumbnail flickering in comment previews

Summary:
Ref T10262. Currently, we always render a tag like this when you `{F123}` an image in remarkup:

```
<img src="/xform/preview/abcdef/" />
```

This either generates the preview or redirects to an existing preview. This is a good behavior in general, because the preview may take a while to generate and we don't want to wait for it to generate on the server side.

However, this flickers a lot in Safari. We might be able to cache this, but we really…