Embed field (similar to "File: ") above embedded objects

This pull request adds an "Embed: " field in the same fashion as images use the "File: " field. The importance of this is that users that do not load remote content (noscript, request policy, text browsers, etc.) cannot see the remote content.

The commit is pretty self-explanatory but you can see this feature in action here:
http://a.pomf.se/gkluex.png

*Note*: This pull request requires a schema update to all posts_* SQL tables. Namely it requires adding an "embed_url" field. I don't know if @ctrlcctrlv has a method to do this easily but if you'd like I can create a migrate script and send it to you.