Create a whitelist for oEmbed displays on the forums

[Clorith]

With the bbPress 2 upgrade, we now have core oEmbed support with every post and topic, but oEmbed these are unreliable and doesn't truly belong there.

Currently most sites linked to are http only, this means we get mixed content warnings and broken embeds to begin with.

There's also no legitimate reason to embed a site on the forums, we're interested in clean links if anything, and embeds break this expectation.

[Otto42]

I'd rather whitelist some set of sites that are okay and potentially useful, such as imgur and such. Objections?

[ocean90]

Maybe ​auto discovery should get disabled for the forums.

[Clorith]

I'm all for a whitelist for useful resources, imgur and the likes is very handy, like when people do screencasts they use youtube, so that one is also quite handy.

[ocean90]

In 4181:

Support Forums: Disable <link> tag discovery for oEmbed.

See #2097.

[SergeyBiryukov]

Related: #2469

[Clorith]

Marking as fixed, as we're no longer appear to do discoveries on URLs posted to the forums.