Thank you page embed support

Hi, I have been facing an issue with embedded form on third party website - the thank you page is not displayed in embed mode and displays full Plone layout. I have created a discussion thread about it as well https://community.plone.org/t/ploneformgen-embedded-view-in-iframe/311

This pull request adds @@embedded view for Thank you page and uses it in case of form is displayed in the embedded mode. The same for fg_result_view template - I added it's embedded version.

What do you think about this solution? 

btw, I was not able to run tests :( so I changed test file only where I think it should do what's described there.