Rich Media support, part 2.

This commit finishes off rich media support in the backend.

• /api/rich_media/parse endpoint is removed.
• MastodonAPI statuses now have card filled when the information is already available.
• TwitterAPI statuses also now have card filled when the information is already available. Loading statuses will block while the URL is crawled if necessary.
• A try/rescue has been handled to clean up exceptions with the OGP crawler.

Things left to do:

• Crawl new posts as they come in if they are likely to appear on a user timeline.
• Crawl posts immediately when the status is created by a local user.