Added background image to landing page.

Actually they are different images at different sizes (following bootstrap responsive breakpoints). The idea is to serve a smaller file if possible for smaller screens.
Moved background CSS rules to a different partial. Devise views also show background (except Edit Registration view).
Reduced jpg quality to reduce background image size in bytes.
WIP: pretty background for landing page and devise views.
Removed original-size background image from the git repo.

It was quite big and not really necessary in the project. It forced the CI environment to download a big file every time it ran tests, with no need.
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.