New site setting: `embed_any_origin` that will send postMessages to
wildcard origins `*` instead of the referer.
Most of the time you won't want to do this, so the setting is default to
`false`. However, there are certain situations where you want to allow
embedding to send post messages when there is no HTTP REFERER.
For example, if you created a native mobile app and you wanted to embed a list
of Discourse topics as HTML. In the code your HTML would be a
static file/string, which would not be able to send a referer. In this
case, the site setting will allow the embed to work.
From a security standpoint we currently only use `postMessage` to send
data about the size of the HTML document and scroll position, so it
should be enable if required with minimal security ramifications.
1cebe7670a