Allow a subreddit's styles to be used for embedded threads.

Adding ?stylesr={srname} to the /live/whatever/embed endpoint adds that
subreddit's stylesheet to the embed.  This allows for custom theming of
embedded live threads.
Add high-selectivity margin setting for .content.

Subreddit stylesheets commonly add an extra margin at the top of
.content to allow room for an announcement link or the like. By
increasing the selectivity this way, we override the common case without
making it impossible to override if it's really desired.
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.