Improve user interface for pages under embargo

API: Clear embargo fields once embargo processed

BUG: User allowed to publish despite embargo
* Hides publish button when embargo fields filled
* Displays a message when site has an embargo

Screen shots:

![embargo_withdate](https://f.cloud.github.com/assets/984753/952478/dcb4cc30-03cf-11e3-9bcf-83e8f0425c28.png)
![embargo_today](https://f.cloud.github.com/assets/984753/952479/e7bb2584-03cf-11e3-822f-b1d0f41e0347.png)