Overhaul the on-reddit embed to look more natural.

This does a few things:

* takes advantage of the new unsandboxed version of mediaembeds from
  r2 to remove one unnecessary level of <iframe>s.
* styles the embed to look more like a regular old self post.
* applies the current subreddit's stylesheet to the embed to allow
  the embed to be in-theme for a subreddit.