Only enforce OEmbed whitelisting for dangerous types

[markjaquith]

Of the four OEmbed response types, only two (video and rich) are potentially dangerous. We should be able to allow all photo and link responses, without a whitelist.

I'm not sure that we can consider photo responses safe, in the sense that we use the value of the url parameter as the source for an image element.

In the past there have been security exploits (such as the GDI exploit) that used image files to trick clients into executing code.

Even the best-case scenario allows the remote server to set and read cookies.

@ryan, @westi, @azaozz, can you weigh in here? If this is going to get in, needs a patch asap to beat freeze. @filosofo's counter-argument sounds reasonable, though.

No patch, and we're entering beta. Punting.