Add support for cards

[CaptainSpam]

I completely forgot that a card could be added in with a status if Mastodon figures there's card data to display for, say, a link or so. Not only do I have to figure out the best way to render all the possible fields in the card, but I also need to integrate the other media types in as need be (see #1, #2, and #3; static image support is already in play).

[CaptainSpam]

One of the fields in a card is, terrifyingly, HTML. As in, a block of HTML that just gets plopped into the card.

Given some of these cards are specific to certain media types (i.e. YouTube embeds), I can't just run said HTML through my JS sanitizer, as those would need JS to actually function. This is troublesome.

I'd imagine my best bet would be to either give up on card support or find some way to render them without the HTML, and put HTML rendering in as an option that defaults to false. Either way, this issue is seriously low priority.