Core Twitter embed:
- Fixed twttr undefined error.
- Made sure embeds still work when another Twitter widget is included in page (e.g., in a pocket).
- Embed cards will now immediately display when posted, instead of on page refresh.
- Embed cards will transition in.
- Refactored code.
Merge branch 'stage' of github.com:vanilla/vanilla into feature/twitter-embed-fix
Merge branch 'feature/twitter-embed-fix' into stage

Conflicts:
	index.php