Hide or mask images embedded in HTML

[evanp]

Images embedded in the HTML with a src on another server can give a poor experience if that server goes down. They can also be a privacy threat, using third-party cookies or IP logging to track remote users.

I see a couple of options here:

1. Replace images in content with proxy URLs, as we do with images outside of HTML content.
2. Hide images until a user clicks a "show images" link. I think email systems do this quite a bit.

There might be other options.

[strugee]

We should proxy URLs. That'll provide a more reliable experience if a remote server goes down, it avoids leaking metadata and it will also fix passive mixed content issues.