Prettier landing page with background image.

Acknowledgements are in a separate jumbotron at the bottom of the page, with grey background so they are readable. Social links have been moved from the footer fragment to the main jumbotron.
Prettier devise views with background image (except edit registration, which is part of the app proper).

Acknowledgements footer is not displayed in devise views. Showing it in the landing page is enough, it made devise views too cluttered.
Added "acknowledgments" line to footer.

This makes it clearer for the user what is all that text in the footer.
Added acknowledgement and license information for the background image to the footer.