Shapshot phantomjs rack: save html for debug. use base_href, pass in CSS

Instead of rewriting the html and css, we simply change the doc base.
We are saving both HTML and PNG files now, so that we can debug the html

[#34511335]
https://www.pivotaltracker.com/story/show/34511335
Shapshot application.js: extract snapshot.js send css, send base_url

I removed the snapshot specific code out of application.js and into its
own javascript file.

Also: We now send all the stylesheets in the page, and a base url.

[#34511335]
https://www.pivotaltracker.com/story/show/34511335
snapshot: interactives.html: Put snapshot into the extra's area.

Also include snapshot.js

[#34511335]
https://www.pivotaltracker.com/story/show/34511335