Enhancement: Add "load 3rd party media" setting

[IBBoard]

There are already several external media sources supported (e.g. Instagram, imgur.com, Flickr) and I've proposed more to fit with the Flickr "cards" on the website (e.g. #468). This is good because it lets you see the media inline.

However, my fix for #457 has found a problem with loading external media when the same image is in multiple places (e.g. Flickr link and a Twitter upload in a tweet leads to double images with no obvious fix, because the Twitter upload can be a different size/recompressed/etc). Thinking about it, there are probably also people who don't want every single image from all of these other sites loading inline as well.

What I thought might be helpful is having a "Load 3rd Party Media?" option in the "Tweet" section to allow people to disable the behaviour and only display Twitter images from the JSON inline in the feed.

As the "is_media_candidate" function in InlineMediaDownloader already has an IF block for the image size then I think the implementation should be as simple as grouping everything except twimg.com and the /photo/1 rules into a group that is ANDed with Settings.load_3rd_party_media() or similar.

I know Corebird tries to stay minimal on the config, but this seems like a setting that could be quite beneficial (since Twitter web interface doesn't do Flickr cards in the main feed until you expand the tweet).