WIP: Implement Mastodon-style embeds

As of now the app relies on Open Graph meta tags and oEmbed, but the plan is to implement Mastodon-style embeds, like what [FxEmbed/fixupx](https://github.com/FxEmbed/FxEmbed), [BetterTwitFix/vxtwitter](https://github.com/dylanpdx/BetterTwitFix), [InstaFix/ddinstagram](https://github.com/Wikidepia/InstaFix), etc. do.